H11C-0764
Fog Collection and its Variability in the Andean Mountain Range of Colombia: A Possible Source for Water Supply?
Hydrological droughts occur naturally in some regions of the Colombian Andean watersheds, some of them associated with large-scale climatic phenomena like El Nino. The associated water scarcity is aggravated by an advancing deforestation of the native highland forests. These events have negative consequences for the environment and for human development. Low water availability in arid/semiarid regions and water scarcity in surface sources in mountainous zones could be managed partially by means of water from fog collection. To date, fog collection variability and potential use of fog water as an alternative for water supply has not been evaluated in Colombia. This study evaluates the spatial and temporal fog collection variability and analyses the potential use of fog as an alternative source of water supply in an Andean rural region of southern Colombia, which is highly affected by droughts and low water availability. Fog collection experiments were carried out, and data collection covered both dry and rainy seasons in the period 2003 - 2005, with daily data registration of fog collection and drizzle/precipitation. Twelve Standard Fog Collectors (SFC), built from polypropylene mesh with a vertical collection surface of 1.0 m2, were installed in a mountainous zone with an area of approximately 500 ha, ranging from 1,680 to 1,850 m a.s.l. Chilean meshes with several shade coefficients were tested. In order to assess the spatial fog variability, isolines of fog-water collection rates were estimated using Kriging as the interpolation method. Our results suggest a high potential for the use of fog to supply domestic water requirements in rural areas. Also, the observed collection yields are consistent with some experimental results of fog harvesting from other countries in South America. Annual average collection rates amounted to 4.2 l/m2/day for precipitation + fog, and 3.3 l/m2/day for fog only. The temporal analysis indicates that the most important month for fog collection was June with 5.3 l/m2/day for rainfall + fog, and 5.0 l/m2/day for fog only in dry days. Finally, it was observed that the tested fog collectors can be also effective devices to collect rainwater, mainly in dry seasons.
H11C-0765
Developing a Framework for Testing Distributed Hydrologic Models at the Catchment Scale – Examples of Test Model Runs for Event Based and Long Term Simulations
We develop a testing framework for distributed hydrologic models at the catchment scale using the hypothetical reality concept. The hypothetical reality, considered an error-free hydrologic response modeled after the 10.5-ha Tarrawarra catchment in Australia, is generated using the sophisticated Integrated Hydrology Model (InHM) representing fully coupled 3D variably saturated subsurface and 2D surface flow with finite-element discretization. The hypothetical realty consists in a data set composed of two subsets designed to test long-term and event-based behavior of the test model. The first subset, a long term data set, comprises an 11-year variable time step time series of hydrograph output and head and saturation levels at 55 observation nodes as well as daily snapshots of head and saturation levels at all nodes in the domain. The second subset, short term data set, has the same type of output, but the domain snapshots are at a much finer time scale, every half hour for the selected rain events. Both data sets were obtained with the same InHM configuration but different output time steps. We use MODHMS (HydroGeoLogic, Inc.), a MODFLOW-based code that solves Richards equation for the 3-D variably saturated subsurface flow and the diffuse wave approximation 2D overland flow with finite differences in a coupled approach as the test model. We present examples of model testing scenarios with variations in spatial discretization, initial conditions, and representation of hydrologic processes. Future work includes testing of model parameters and soil characteristics, with the test model running first without any calibration and then with calibration against the hypothetical reality.
H11C-0766
Hydrological Features on Subalpine Forest Zone in the East of Qinghai-Tibet Plateau
The Hengduan mountain chains of China is situated on the east of Qinghai-Tibet Plateau with area of more than 400,000 km2. Mountains and rivers run through in north-south direction, and are collocated side by side on east-west. Elevation difference between ridges and valleys has great disparity, normally of 1000-2500m, so the vertical zones of vegetation are very distinct. Subalpine coniferous forest zone, mainly composed of fir (Abies) and spruce (Picea), is on altitude of 2800-4200m, which is a chief component of the forested area in southwest China, and an important region for water conservation of several international rivers inlcuding Nujiang River and Lancangjiang River, as well as the world-famous Changjiang River. Thus, it has both theoretical and practical significance to study hydrological process and laws of forest in this region. The study area is located at the Gongga Mountain, on the east edge of the Hengduan mountain chains. Elevation of the main peak is 7556m, and elevation difference between ridge and valley on the eastern slope is 6400m. An ecological observation station was built at altitude of 3000m on the eastern slope of Gongga Mountain in 1988, mainly for alpine ecology and forest hydrology research. Based on the analysis of 20- years observation data from this station, it is revealed that hydrological process of forest in this area has several features as follows: (1) Canopy interception of primitive fir (Abies) forest is obviously greater than other tree species, and interception rate is 30-40%. Maximal canopy interception of one-time precipitation of primitive fir forest is commonly 2-5mm. According to observation data of canopy interception, a conceptual model of canopy interception of fir forest is established: R=1.69[(1-exp(-0.41P))+0.19P (P is precipitation in mm); (2) Natural valid moisture holding capacity in layer of moss¨Cdecayed wood and leaves beneath trees is up to 5.6mm. Porosity in soil surface layer and non-capillary porosity are 95% and 12%, respectively. Infiltration equation of topsoil, which has high permeability of soil moisture and matrix of moraine, is: f=7.29+5.40exp(-0.06t)(f in mm;t in minute);(3) River runoff in study area is chiefly supplied by interflow and underground flow. Underground flow is divided into shallow rapid underground flow and deep slow underground flow. Their confluence times are 5-15 days and 2-3 months, respectively. It is obvious that this wooded area possesses strong function for water conservation;(4) Through analysis on water movement process and temporal and special distribution of hydrological layers such as canopy, ground flora, soil, etc., we establish the 'Distributed Hydrological Model in the Watershed' for subalpine forest zone. By simulation of this model, it is found that when forest coverage is 35-40%, hydrological regulation function of subalpine forest ecosystem reaches the best conditions. This result provides theoretical basis for sustainable operation of forest resources as well as water conservation and utilization of subalpine forest zone in the Hengduan Mountain area.
H11C-0767
Comparative Ecohydrologic Effects of Forest and Pasture on Water Balances in Kona, HI
To understand the possible hydrologic impacts of interconversion between forest and pasture in a tropical ecosystem, we looked at vegetation interactions with precipitation and at comparative water use by forest and pasture plants. We gathered micrometeorological data from four sites in the mountains on the leeward side of Hawai'i Island over 20 months. Throughfall varied dramatically within and between sites; at some locations it was greater than rainfall, suggesting an important role for cloud interception. Stemflow played a small but measurable role in water input, also likely a result of cloud interception. Because of very high relative humidity and low wind speeds in this convective rainfall system, solar radiation is the main driver of evapotranspiration and the water-use characteristics of vegetation play a decisive role. The stomatal conductance of Meterosideros polymorpha, the dominant forest tree, is low compared to that of the pasture grass, so forests are estimated to evapotranspire at half the rate of pastures.
H11C-0768
Linkages between hillslopes and channels: spatial pattern of infiltration and connectivity of overland flow in a headwater catchment draining Japanese cypress forest, central Japan
Linkages between hillslopes and channels are essential theme for understanding overland flow and transfer within headwater catchments. Based on field monitoring in small (0.5 x 2 m) and large (8 x 25 m) plots, scaling effect and connectivity of Hortonian overland flow was examined in steep forested hillslopes with Japanese cypress (Hinoki, Chamecyparis obtusa) plantations with sparse understory vegetation [Gomi et al., 2008 WRR]. We used distributed runoff model based on contour based topographic analysis for investigating overland flow generation in a 0.3 ha headwater basin. Spatial arrangement of infiltration capacities based on the distribution of vegetation and litter ground cover were consider in the model parameter. We also consider changes in infiltration capacity with respect to rainfall intensity and soil water repellency. Further internal spatial variability of infiltration capacity based on standard deviation of estimated infiltration capacity using a rainfall simulator. Observed and estimated overland flow generation suggested that including spatial variability of infiltration capacity improved simulation of overland flow generation.
H11C-0769
Quantifying Rainfall-Partitioning Dynamics with High-Temporal Resolution Measurements for a Temperate Rainforest in Central Taiwan
Quantifying the partitioning of rainfall into throughfall, stemflow, and interception loss by canopies is important to the balance of hydrological cycles in forest. Direct high-temporal resolution measurements of these components were conducted at an experimental watershed of the Lien-Hua-Chih Research Center in central Taiwan. The vegetation type is a temperate rainforest, which is commonly seen in low altitude mountainous areas in Taiwan, with a mixture of evergreen hardwood having an averaged canopy height of 17 m and sparse pteridophytes in the lower canopy. A trapezoidal pilot plot with an area of 42 m2 was established in 2008 adjacent to a meteorological tower of 22 m tall. The crown projection area accounts for 61% of the plot area. The leaf area index measured by LI-COR LAI-2000 around and inside the plot varied from 3.4 to 4.6. Throughfall was measured by six tipping bucket rain gauges evenly placed over the plot. Steamflow was measured on two trees (0.21 m and 0.32 m DBH) accounted for 53% of the total crown projection area. Stemflow water was collected in plastic collars fixed by silicon sealant around the stems and measured by tipping bucket rain gauges. An eddy covariance (EC) system, including a 3-D sonic anemometer (Young 81000) and a krypton hygrometer (Campbell KH20), installed at 25 m height provided direct measurements of evaporation from interception loss after rainfall ceased. However, the contribution of transpiration and soil evaporation to EC measurements under wet canopy conditions requires further investigations. Excluding extreme typhoon rainfalls, a total of 23 rainfall events with durations ranging from 20 to 435 minutes from April to August in 2008 were investigated. Throughfall and stemflow account for 74.7% and 15%, respectively, of the total precipitation. The residual 10.3% of rainfall partitions is attributed to interception loss which is comparable with those measured by the EC system for cases of fully saturated canopy. Interception loss was further simulated by the revised Gash analytic model (Gash et al., 1995) and the Rutter type model (Keim and Skaugset, 2004). Although overall efficiencies of both models are acceptable, both models overestimated the total interception loss.
H11C-0770
Quantifying the Interactions Between Spatially Variable Throughfall and Subsurface Connectivity at the Hillslope Scale
Forested hillslopes exhibit a baffling array of heterogeneity in landscape properties and complexity of their responses to fixed hillslope attributes (e.g. slope, soil depth etc.) and temporally varying throughfall. One widely reported spatio-temporal pattern of hydrological response at the hillslope scale is the threshold nature of subsurface stormflow production. In many upland systems, this is linked to the development of connected patches of subsurface saturation at the soil-bedrock interface - a pre-condition for the initiation of lateral flow at many experimental sites examined around the world. To date however, these findings have been largely based on case studies of individual hillslopes with little synthesis among and across sites. As a result, the hydrology of forested hillslopes still lacks the compact organization of empirical data and observations of hillslope responses that might facilitate extrapolation to and prediction of hillslope behavior in ungauged hillslopes. One pressing issue that is difficult to assess with single-realization hillslope ecohydrological studies is how spatially varying throughfall inputs conspire with the subsurface topographic control on the development of connected patches of subsurface saturation necessary to drive flow. Here we describe a new study where we examine how patterns of measured throughfall, applied to a hillslope with variable soil depth and irregular subsurface topography, influence the spatio-temporal hydrologic response of the hillslope to a storm event. We present a number of virtual experiments using Hydrus-3D, a Richards equation-based finite element model. Topography and hydrologic field observations from an existing study hillslope were used to calibrate and test the model. The analysis of spatio-temporal patterns of subsurface moisture demonstrates how different realizations of throughfall patterns interact with the underlying subsurface topography, controlling the formation of subsurface saturation connectivity at the soil- bedrock interface and the generation of subsurface stormflow.
H11C-0771
Evaluation of Spatial Patterns of Infiltration Capacity on Storm Runoff in a Forested Watershed: Utilizing of LiDAR Data in a Distributed Runoff Model
Spatial pattern of infiltration capacity is important for understanding of Hortonian overland flow (HOF) generation and runoff processes. We coupled a distributed runoff model and LiDAR data analysis for evaluating effects of spatial patterns of infiltration capacity on the contribution of HOF to storm runoff in a forested watershed. Because infiltration capacity, ground cover of understory and litter, and relative light intensity are interrelated, we first estimated relative light intensity of each 10 x 10 m grid. The values were converted into ground cover classes using an empirical relationship. Then, infiltration capacity was estimated for each grid (5.3 – 137 mm h-1) based on a relationship between ground cover class and infiltration capacity obtained by rainfall experiments. Estimated spatial patterns of infiltration capacity were applied to the model by topographic analysis. We simulated storm runoff and the contribution of HOF at the outlets of the watershed (4.9 ha) and three nested watersheds. By applying the spatial patterns of infiltration capacity, simulated storm runoff generally agreed with observed runoff. The contribution of HOF to storm runoff was negatively correlated with percentage of area where the relative light intensity was greater than 5%. The method proposed in this study is useful for evaluating contribution of HOF to storm runoff in larger watersheds.
H11C-0772
Parameter Estimation of Hydraulic Conductivity in the study of subsurface flow for a very steep hillslope: a study case in Valsassina valley (Italy)
During a rainfall of high intensity, a correct evaluation of pore pressure in soil is fundamental. At the same time, the water flow from the upper side of the hillslope needs to be evaluated properly. In mountain regions, hillslopes are steep, with thin soil layers, high hydraulic conductivities and macroporosities (Mulungu et al., 2005). Water in soil moves downward to an impermeable layer generating the subsurface flow (Chorley, 1978; Sidle et al., 1995; Montgomery and Dietrich, 2002; Mulungu et al., 2005). For this reason, the debris flow occurs more frequently in steep, convergent hillslopes (Reneau and Dietrich, 1987; Ellen et al., 1988). In the past decades, the subsurface flow has been represented by models with different complexities (Beven and Kirkby, 1979; Fipps and Skaggs, 1989; Paniconi and Wood, 1993; Verhoest and Troch, 2000; Ogden and Watts, 2000; Ebel et al., 2007). Recent literature points out that simple, but physically based models, are required, able to characterize the hydrological processes both at local and catchment scale. On one side, these models are required to treat the spatial and temporal variability of the hydrologic, geomorphologic and topographic input accurately, on the other side the same models should be simple and without excessive computational effort. The HsB model (Troch et al., 2003) seems to match these requirements. The model has adopted to simulate the subsurface flow process in a divergent hillslope in Valsassina Valley, in northern Italy. The capability of the model has been tested comparing the water table levels and the outflow on the lower side with the values obtained with a 3D numerical scheme, based on Richards equation. Furthermore, in this study case, the hydraulic conductivities calculated adopting the PTFs (Saxton et al., 1986) have been demonstrated not to represent the hydraulic characteristic of the hillslope properly.
H11C-0773
Calibration of Rainfall-Runoff Model by Referring to Hydrological Separation of Runoff Components using Chemical and Isotopic Characteristics of Discharge
Calibration of Rainfall-Runoff Model by Referring to Hydrological Separation of Runoff Components using Chemical and Isotopic Characteristics of Discharge Hidetaka Chikamori Graduate School of Environmental Science, Okayama University, JAPAN A rainfall-runoff model is generally calibrated by minimizing error in calculated runoff using records of hydrological components, that is, observed rainfall, discharge and observed or estimated evapotranpiration. However, calibration using only hydrological components sometimes produces a model with strange structure that does not reflect physical properties of an objective basin. It is probably due to error in referred hydrological records. In this study, the author calibrated a rainfall runoff model using not only hydrological record but also chemical and isotopic data of discharge so as to obtain a reasonably structured model from multiple viewpoints. Besides, the model structure was improved in order to simulate isotopic characteristics well. It is well known that ratio of surface flow in total flow can be estimated by change in concentration of cation or anion. Relative concentration of 18O, δ18O is well used for separating runoff of retained water in soil as "old water" from total runoff. A Long-and-Short Term Tank Model (LST2 Model) was applied to three Hinoki Cypress catchments in Mie experimental basin located in the middle of Japan. One of these catchments is of well-maintained planted forest, and two are of poor-maintained planted. A model was calibrated by Differential Evolution for each catchment using hydrological data, concentration of K+ and δ18O. In these catchments, Gomi et al (2008) showed that concentration of K+ well expresses ratio of surface runoff to total runoff, and that δ18O subsurface runoff to total runoff. The results show that an original version of LST2 Model cannot simulated delayed subsurface flow ratio estimated by δ18O, although it well simulates surface flow ratio estimated by concentration of K+. For overcoming this weakness, a LST2 Model was modified by installing a water retention layer storing soil water to each tank, and the modified LST2 model performed well on simulating runoff so that not only surface flow ratio but also subsurface flow ratio were well identified with estimated by chemical and isotopic
H11C-0774
Effects of Spatially Heterogeneous Vegetation Water Stress on Hydrological Processes within a Forested, Mountain Watershed
Vegetation water stress plays an important role in the movement of water through the soil-plant-atmosphere continuum. However, the effects of water stress on the hydrologic balance of an entire forested watershed remain poorly understood, due in part to spatially and temporally heterogeneous micrometeorological and ecophysiological conditions within the watershed. We investigated the impact of vegetation water stress on the hydrologic balance of a small (3 km2) forested watershed in the northern Rocky Mountains (Montana, USA) using a spatially distributed ecohydrological model. Topography and vegetation input variables were derived from high resolution airborne lidar, and surface-based observations provided atmospheric input variables. We performed model simulations of spatially and temporally heterogeneous water stress and hydrological fluxes for a single growing season following snow melt. We determined topographic, vegetation, and micrometeorological controls on the temporal and spatial variability of modeled water stress and evaluated the extent to which heterogeneous water stress affected evapotranspiration and stream discharge within the watershed.
H11C-0775
Evaluation of Surface Hydrological Connectivity Between a Forested Coastal Wetland and Regulated Waters of the United States
Rapid urbanization, industry, and agriculture have put enormous developmental pressure on coastal forested wetlands along the Texas coast. At least 97,000 acres of freshwater forested wetlands on the Texas coast have been lost since 1955, amid much larger losses of other coastal wetland types (TPWD-Texas Wetlands Conservation Plan, 1996). Some coastal wetlands are protected by federal regulations under the Clean Water Act in an effort to maintain wetland hydrological and ecological services, such as water quality improvement and flood control. However, federal protection of many important coastal wetlands is dependent upon documented proof of a hydrologic connection to federally protected Waters of the United States and reasonable influence on the quality of those waters. This study focuses on a 13 acre catchment of coastal flatwoods wetland with an ambiguous legal status because of a possible , but undocumented, hydrologic connection to regulated Waters of the United States. Documentation of the hydrologic connectivity of this type of wetland is critical because of the geographic extent of similar wetlands and their contributions to water quality. The objective of the study was to determine if a hydrologic connection exists, and if so, to quantify the strength of the connection. A surface connection was established based on runoff and rainfall data collected since April of 2005, with the wetland discharging surface water directly into an adjacent protected wetland. The connection was weak during dry years, but in years with average rainfall, surface runoff accounted for a much more significant portion of the water budget. These results suggest that runoff water from similar wetlands contributes directly to protected wetland waters, and may influence water quality downstream.
H11C-0776
Change in Flow Characteristics of a Forest Catchment due to Thinning
Man-made forests of Japanese cedar widely cover mountainous land in Japan. Recently, the unthinned man- made forests with poor floor vegetation are rapidly increasing due to decline of forest industry and there is an opinion that the thinning in such unsound forests should be carried out in order to recover the function of soil and water conservation. But, research reports discussed hydrological effects of thinning is still not many by comparing with a lot of reports on effects of clear cutting. This study discusses change in flow characteristics of a forest catchment due to thinning for the quantitative evaluation of hydrological effects of thinning. Study site is a small experimental catchment of 0.34ha mainly covered by unthinned man-made forest of Japanese cedar located in eastern area of Shimane prefecture, Japan. Precipitation and runoff discharge at catchment outlet were observed by Shimane University and hydrological data of eight years from December, 1998 through December, 2006 are available. In this catchment, thinning was done in April, 2004 and 32% of trees were cut. In this study, a conceptual rainfall-runoff model developed by Fukushima (Hydrological Processes, 14-3, 1988) are applied to the catchment and daily flow hydrographs are simulated continuously in both periods before and after thinning. All model parameters are calibrated by using data before thinning and for period after thinning, parameters of interception and transpiration are modified to decrease interception of 30% and transpiration of 26%, respectively and other parameters are not changed. Simulated hydrographs show good agreement with observed ones before and after thinning. The decrease of annual evapotranspiration due to thinning is estimated about 300mm. Moreover, hydrographs assumed no thinning for whole period are also simulated and hydrographs of unthinned and thinned case are compared. The result shows that runoff discharge of thinned case is always larger than that of unthinned case. Therefore, implementation of thinning is advantageous in respect of water resources conservation.
H11C-0777
Spatial and Temporal Variability of Interception Experiments in Luxembourg
From several field experiments we know that canopy interception is highly heterogeneous in space and time.
Depending on the density of the canopy (which changes throughout the seasons), some places intercept
more rain water than others. Furthermore, it is also possible that due to canopy structure rain water is
funneled into places where throughfall can be even higher than gross precipitation. Since interception is a
threshold process, this has also a large impact on the amount of throughfall that is intercepted by the forest
floor. Combined with the spatial distribution of the leaves on the forest floor this can result in a high spatial
distribution of the infiltrated water.
To investigate the spatial and temporal variability we measure both canopy and forest floor interception in a
beech forest in the Huewelerbach catchment in Luxembourg. Canopy interception is measured spatially with
a dense network of throughfall gauges and is calculated by subtracting throughfall and stemflow from gross
precipitation. Forest floor interception is measured on point scale with a special developed device. It consists
of two basins which are mounted above each other. The upper basin is permeable and contains the forest
floor. By weighing both basins continuously, interception from the forest floor can be calculated.
The spatial correlation is determined with semi-variograms. It appears that on average within 6 meters the
throughfall is spatially correlated. As expected this range changes throughout the seasons with the canopy
development. In winter time the range is circa 7 meters and in summer time about 5 meters. The time stability
plots show that the throughfall data is persistent in time and that there exist clear dry and especially clear wet
throughfall collectors. There is no significant difference in the stability plots of the different seasons.
http://www.interception.citg.tudelft.nl
H11C-0778
Causes of Variability in the Effects of Vegetative Ash on Post-Fire Runoff and Erosion
Vegetative ash formed during forest wildfires has varying effects on post-fire runoff and erosion. In some cases the ash layer reduces runoff and erosion by storing rainfall and by protecting the soil surface from surface sealing and rainsplash detachment. In other cases, the ash layer increases runoff and erosion by forming a surface crust, clogging soil pores, and providing a ready source of highly erodible fine material. Since only a handful of studies have measured the hydrogeomorphic effect of ash, it is unclear whether the observed variability in its effect reflects initial spatial variability in the ash properties due to factors such as fuel type and fire severity, or differences that develop over time due to compaction and erosion or exposure of the ash to rainfall and air. The goal of our research was to determine if the observed differences in the effect of ash on runoff and erosion are due to: 1) variability in initial ash hydrologic properties due to differences in combustion temperature and fuel type, or 2) variability in ash hydrologic properties caused by mineralogical phase changes that develop after the ash is exposed to water. We created ash in the laboratory using wood and needles of Lodgepole pine (Pinus contorta), Ponderosa pine (Pinus Ponderosa) and Douglas fir (Pseudotsuga menziesii) and at 100° C temperature increments from 300 to 900° C. A subsample of ash from each fuel type / temperature combination was saturated, left undisturbed for 24 hours and then oven dried at 104° C. Dry and wetted ash samples were characterized in terms of: structure (using a scanning electron microscope), carbon content, mineralogy (using X-ray diffraction), porosity, water retention properties and hydraulic conductivity. Ash produced at the higher combustion temperatures from all three fuel types contained lime (CaO), which on wetting was transformed to portlandite (Ca(OH)2) and calcite (CaCO3). This mineralogical transformation resulted in irreversible hardening and crusting of the ash, and hardened ash had a significantly lower hydraulic conductivity than unhardened ash. Ash produced by high severity fires may undergo this same hardening and crusting process after it is wetted by rainfall whereas ash produced by lower severity fires will not, and this may explain in part the contrasting hydrogeomorphic effects of ash that have been reported in the literature.
H11C-0779
Variations in Soil Evaporation and its Contribution to the Water Balance of a Semi-arid Forest
Variability of components and processes is present in all forests but is enhanced at dry ones due to the contrasting differences between shaded and exposed areas and the large amplitudes of diurnal, seasonal and inter-annual weather conditions. In such ecosystems, soil evaporation (E) is expected to have large variability and account for a significant part of the total evapotranspiration flux (ET), therefore important for the understanding of processes such as water use, respiration and ecosystem productivity. We report on a four years study (2003-2007) in the Yatir semi-arid pine forest in Southern Israel (40 years old P. halepensis; LAI=1.5; mean precipitation 280 mm/yr). E was directly measured using a modified soil respiration chamber (LI-COR) on 14 permanent soil collars in sites covering the forest spatial variability, carried out on a weekly basis during the research period. Field-averaged E was compared to eddy flux measurements of ET, heat-pulse measurements of tree transpiration (T) and environmental measurements (precipitation, P; soil water content, SWC; radiation, measured as PAR). Our results showed large spatial variability in E (daily SD between sites ~47%), with fluxes measured at exposed areas doubled that of shaded areas, on average. Variability was found to be correlated with PAR (up to 92% higher in exposed compared to shaded sites) and SWC (which was higher in exposed areas during the wetting seasons but higher in shaded areas during the drying seasons). The proportion of forest floor shaded fraction was shown to be a function of tree height, canopy width and tree density. Differential seasonal patterns in E and T were observed: E peaked twice - during early and late winter (up to 0.80 mm/day) and T peaked in spring (up to 1.20 mm/d; coinciding with maximum SWC in the root zone). Low E (0.10 mm/day) was measured during mid winter (max daily temp below 15°) and low E and T in summer (SWC below 10%). The E/ET ratio varied seasonally and on an annual basis E accounted for 44% of ET (102±8 mm). The results indicated that E was a significant part of the hydrological budget in this forest. Simulations based on quantitative relations between E and canopy structure developed here could provide a simple predictive/management tool to optimize tree water use and efficiency.
H11C-0780
Changes In Snowmelt Timing In Response To Pine Beetle Infestation In Lodgepole Pines In The Colorado Rockies
Since 1996, roughly 1.5 million acres of lodgepole pine forest in Colorado have been infested by mountain pine beetles (Dendroctonus ponderosae). We measured physical snowpack properties (depth, density, and temperature) under stands of both living and dead lodgepole pines in the Colorado Rockies. This data allowed us to investigate the effect of increased forest canopy transmittance due to tree death on potential advances in the annual hydrograph. We compared snow accumulation and melt on north-facing and south- facing slopes at an elevation of approximately 3000m. As expected, topography-dominated solar forcing is the chief factor in snowmelt: snow on south-facing slopes melted earlier in the season than north-facing slopes. Comparing stands of dead and live trees within topographic zones revealed a few dramatic differences: snow water equivalent was lower and mean snowpack temperature was warmer in dead lodgepole pine stands. Temperature timeseries from within the snowpack suggest that snow in dead tree stands became isothermal sooner than snow in living tree stands. Together these show that there was indeed earlier snowmelt in lodgepole pine forest regions infested with mountain pine beetle. Earlier snowmelt will likely cause peak snowmelt discharge to occur sooner.
H11C-0781
Spatial and Temporal Variability of Hydraulic Conductivity on Forested Mountain Hillslopes
Soil hydraulic conductivity is often assumed spatially and temporally stationary, but steep forested hillslopes can exhibit high variability in soil properties. We investigate hydraulic properties of soils on mountain hillslopes in a seasonally snow-covered conifer forest to improve understanding of how this variability affects hillslope subsurface flow and ecosystem response. The hillslopes are at 2900m elevation within the Fraser Experimental Forest in Colorado, USA. Two methods are used to determine hydraulic conductivity for the hillslopes: (1) tension infiltrometer measurements, and (2) inverse model simulations. For the first method, a Decagon Mini-disk infiltrometer was used to measure surface hydraulic conductivity at 18 locations. Measurements were collected during summer 2008 twice weekly from May to August. Initial results from field measurements show hydraulic conductivity values ranging from 0.036cm-hr-1 to 3.6cm-hr-1 at 1cm of tension. Infiltrometer values show wider variation in late spring after snowmelt than later in the summer. For the second method, inverse simulations were set up using the variably saturated flow model, HYDRUS 1-D, which simulates vertical soil moisture movement. Inverse simulations identified three parameters of the unsaturated hydraulic conductivity function including saturated hydraulic conductivity. Inverse simulations were run for the peak snowmelt time period, from May 1st to May 31st, at an hourly time step. The inverse simulation objectives were to minimize errors between modeled and measured soil water contents at 5 and 50 cm depth. Soil moisture was measured in situ at these depths with Vitel Hydra Probes. To force the simulation, snowmelt fluxes were added at the ground surface via an atmospheric boundary condition. Melt fluxes were derived from acoustic snow depth sensors and snow pit measurements of snow density. Results of the HYDRUS 1-D inverse simulations give hydraulic conductivity values ranging from 4.3cm-hr-1 to 13.7cm-hr-1 at 1cm tension. These values are at the upper end of the hydraulic conductivity values measured with the infiltrometer, and the model-derived hydraulic conductivity estimates have lower overall variability than the infiltrometer measurements. Because the inverse simulations represent a larger depth of the soil profile, the domain represented may incorporate more preferential flow or high permeability pathways than would be measured with a tension infiltrometer. At a larger scale of integration, these faster pathways become more dominant than for smaller-scale infiltrometer measurements.
H11C-0782
Soil moisture and precipitation thresholds: implications for hillslope contributions to runoff
Subsurface runoff from hillslopes is widely recognized as an important contributor to stream flow generation; however, processes that control how, when, and to what extent hillslopes connect to streams remain unclear. In a small glaciated catchment at the Hubbard Brook Experimental Forest in New Hampshire, we instrumented the hydrologic control catchment (WS3) with a distributed well network and a ridge-to-stream transect of soil moisture sensors to continuously monitor shallow groundwater and soil moisture dynamics. Multiple independent measurements of hydraulic conductivity were used to estimate lateral subsurface fluxes through the near-stream aquifer and determine theoretical runoff contributing areas during storm events. We identified a strong threshold relationship between stormflow and the combination of total stored pre-event soil water and gross precipitation as well as the maximum groundwater height during an event. Our results suggest that this threshold represented a significant change in contributing area. Below the threshold, the runoff contributing area was confined to the near-stream zones, while above the threshold, the contributing area expanded laterally onto neighboring hillslopes. The contributing area expansion established a stream/riparian/hillslope connection in each of our instrument transects, which during the dormant season, frequently persisted until the next runoff event. This study emphasizes the importance of soil moisture storage nonlinearities as they relate to spatial and temporal patterns of saturated zone dynamics, hydrologic connectivity, and catchment-scale runoff response.
H11C-0783
Comparison of Soil Moisture Patterns Between Conifer and Aspen Hillslopes in Northern Utah in Low and Average Precipitation Years
The study watershed, located in Northern Utah, receives most of its precipitation as snow. On forested hillslopes, spring snowmelt and resultant soil moisture patterns often differ between deciduous aspen and conifer stands. In this study, hydrologic connectivity between hillslopes and stream was studied by measuring soil moisture profiles along a transect and surface soil moisture distribution in 10 m by 20 m grids, in two consecutive water years with low and average winter precipitation. Average snow water equivalents were similar between aspen and conifer stands but a greater variability of snow pack was found in the conifer stand in both years. Melt water input saturated soil profiles in both aspen and conifer stands, but the saturation of soils at 50cm to100cm depth was observed earlier on the aspen hillslope, which this trend was more pronounced in the relatively wet year. Grid measurements indicated that soil moisture in the conifer hillslope never exhibited spatial organization in the top 13 cm soil, while the aspen hillslope showed organization on a scale of 7 to 19 m, indicating potential lateral flow in shallow soil layers.
H11C-0784
The Effects of Ashe Juniper on Groundwater Recharge in the Edwards Aquifer
Understanding groundwater recharge rates has direct relevance for management of the Edwards Aquifer, which serves as the main source of fresh water for the city of San Antonio and surrounding communities. As population around San Antonio continues to grow, so does the demand for water and the stress placed on the aquifer. A method that is commonly believed to augment water yields is brush management. Recently on the Edwards Plateau decreasing streamflow has coincided with increasing juniper density. This has led many to believe that removing juniper would increase available water. Due to its karstic nature, the recharge zone of the Edwards Aquifer is assumed to be a prime location for augmenting water yields through vegetation manipulation. This study assesses the dynamics of recharge and the effects of manipulating surface vegetation. To accomplish this, a shallow cave located in the Edwards Aquifer recharge zone in San Antonio, Texas was instrumented to monitor drip recharge in response to simulated rainfall events. In 2004, simulations were conducted over the cave to measure recharge rates with a dense Ashe juniper canopy. The data and observations from the initial simulations were used to establish a baseline with the juniper in place. In March 2008 the juniper stand was cleared and the rainfall simulations were reproduced in June and July 2008. Results initially indicated that removing the juniper decreases recharge, however, surface runoff was significantly increased. From the results we can conclude that the dynamics of recharge are affected initially following removal of juniper.
H11C-0785
Analyzing the Impacts of Forest Cover Changes on Watershed Hydrology
As an effort to understand the role of North American Forests as the sinks and sources of atmospheric carbon, the geographical distributions of forest disturbances have been analyzed under the North American Carbon Program (NACP). The extent, intensity, and rate of forest changes over last 30 years have been recently mapped, combining the temporal Landsat archive and the national forest inventory data. This study is to investigate the consequences of forest cover changes on watershed hydrology. Using the long term stream flow data collected by the USGS, we will identify and derive meaningful hydrologic indices that can characterize variations in hydrological processes. Detailed statistical analysis will be conducted to relate the temporal trend of the indices to the spatial-temporal variability in forest system at multiple scales. The results will have important implications in understanding how the hydrological system responses to changes in forest dynamics and developing critical forest management policy. The study will be done in the southern states of the USA, Mississippi and Alabama.