H34D-01 INVITED
Atmospheric Moisture Demand Under Global Warming
Warming near the surface increases the water vapor holding capacity of the air. Since the vapor saturation level or relative humidity in the air is fairly stable on annual to longer time scales, this would lead to increased atmospheric demand for water vapor if other things being equal. However, accompanying changes in surface wind speed and solar radiation can alter this general trend under global warming at least on regional scales. Decreasing trends in pan evaporation from the 1950s- early 1990s over many parts of the continents suggest that recent warming did not play a dominant role in determining atmospheric demand of water vapor over many areas. In this talk, I will examine historical data for trends in surface humidity, wind speed, and solar radiation and their implications for potential evapotranspiration over global land. I may also discuss climate-model simulated changes in evapotranspiration if time permits.
H34D-02
Study on Change Trend of Pan Evaporation in Recent Years of China
Based on observational data of 664 meteorological stations in China from 1960 to 2000 and combined with the Geographic Information System of ArcGIS, the spatial and temporal distributions and their trends for pan evaporation in China are investigated. The results indicate that, from 1960 to 2000, the rate of pan evaporation has steadily decreased in China. Compared with the period of 1960s to 1970s, the rate of annual pan evaporation during 1980s to 1990s has decreased 99.8mm. Seasonal analysis show that the decrease of pan evaporation is mainly in spring, summer and winter. Spatial distribution of the rate of change shows that the decrease of pan evaporation is mainly occurred in East China and Central South China. Further analysis show that the decrease of pan evaporation is mainly related to reductions in solar irradiance resulting from sun-shine percentage decrease.
H34D-03
Decreasing Evapotranspiration on the Tibetan Highlands and its Implications for Agriculture
Evapotranspiration (ET) is one of the central factors of the hydrological cycle regulating the moisture transfer from soil and plants to the atmosphere. Feedback between soil moisture and atmospheric humidity, mainly governed by ET, may be directly responsible for variations in the strength of regional circulation of the Asian Monsoon System with the major part of this energy transfer conducted through the "elevated heating surface" of the Tibetan Plateau. No reliable information on spatial and temporal variations of ET rates on the Tibetan Plateau has been available until recently. Penman-Monteith ET estimates of 63 stations located on the Tibetan Plateau between 1591 and 4670 m a.s.l. are presented that show that despite the large altitude potential ET (PET) rates in some parts of the plateau are comparable to those of the South Chinese lowlands at similar latitudes (up to 1300mm a-1). In all seasons the major part of the stations show declining PET and pan evaporation rates that appear to be related to changes in solar radiation and wind speeds. On average PET rates (1961-2000) have decreased by 13.1mm decade-1 with annual trends at individual stations ranging from -79.5 mm to 84.8 mm decade-1. Water balance calculations point to decreasing water availability for agricultural production in the Central Tibetan valleys where most of the agricultural areas are located. In the major part of the Tibetan Plateau decreasing water availability appears to stand in contrast to an anticipated increase of the hydrological cycle under climate change conditions. http://www.staff.uni-mainz.de/thomasa/english/tibet/proj_tibet_03_2003_e.html
H34D-04
Understanding the relationship between actual and potential evapotranspirations from long- term water balance analysis and flux observation
Increase in air temperature and decrease in pan evaporation was found to be common worldwide during the past half century. This results in controversy in view of the changes to the hydrological cycle. Increases in precipitation have been expected due to the Clausius¡§CClapyeron relation in that the specific humidity increases exponentially with the greenhouse-gas induced temperature increasing and confirmed by measurements over northern extratropical land areas. The hydrologic cycle is expected to be intensified (or accelerated). However, the decreased pan evaporation is found to be well related to the global dimming, i.e., the decreased solar radiation induced by the pollution increasing, thus evaporation (i.e., the latent heat flux) should be steadily decreasing from the energy balance perspective. Many researchers explained that the potential evaporation (usually measured by pan) is decreased with increasing of precipitation; however, the increased soil moisture (due to precipitation increasing) can be evaporated because of extra energy available. Therefore, the actual and potential evaporation are in complementary relationship, which is expected to unify the controversy between global warming and dimming. This means that pan evaporation decrease implicates acceleration of the global hydrologic cycle, i.e., increase in the terrestrial evaporation. Based on the complementary theory, many operational formulae have been introduced to estimated actual evaporation from the potential evaporation. Our recent water balance analysis of 108 catchments in non-humid regions of China has shown that there are no general opposite trends between potential and actual evaporation in the same period. A novel phenomenon has been found that the complementary relationships in evaporation are distinctly confirmed when the annual actual and potential evaporation are plotted against annual precipitation; However, complementary relationships disappear in many catchments when actual and potential evaporations are plotted against the time (year) during the same period. This means that complementary idea cannot provide universally correct predictions on the trend of actual evaporation only from the potential one. In this research, we examine the coupled water-energy balance based on Budyko hypothesis and proposed a conceptual model for predicting the inter-annual variability of annual water balance, and the change trends of water balances due to climate changes. The wet environment evaporation was defined as the boundary condition in the Bouchet hypothesis and introduced into complementary relationship (CR), which combined the actual evaporation with potential evaporation in an equation. However, the CR was derived in a closed system where no horizontal energy advection existed. The effect of the horizontal advection on the CR in a real open system was also analyzed in this study. Using the long-term water balance analysis in the 108 study catchments and flux observation at 7 sites in Asia monsoon region, the regional and seasonal variability of the complementary relationship was examined. Key Words: climate change, evapotranspiration, water balance, flux observation, Budyko hypothesis, Bouchet hypothesis
H34D-05
Pan Evaporation Measurements to Estimate Actual Evaporation
Pan evaporation as a form of apparent potential evaporation (PEA) can be employed to infer actual evaporation (E). With PEA measurements the Complementary Relationship of evaporation becomes asymmetric, the degree of which can be estimated as a simple function of the air temperature. An extended and modified version of the Advection-Aridity (AA) model is formulated to estimate E either from PEA or true potential evaporation measures. New findings explain important features of the Morton model and show that the modified AA model improves upon Morton's evaporation estimates.
H34D-06
Projected increase in continental runoff due to plant responses to increasing carbon dioxide
In addition to influencing climatic conditions directly through radiative forcing, increasing carbon dioxide concentration influences the climate system through its effects on plant physiology. Plant stomata generally open less widely under increased carbon dioxide concentration, which reduces transpiration, and thus leaves more water at the land surface. This driver of change in the climate system, which we term ‘physiological forcing', has been detected in observational records of increasing average continental runoff over the twentieth century. Here we use an ensemble of experiments with a global climate model that includes a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff, in the context of uncertainties in future precipitation. We find that the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by 6 per cent relative to pre-industrial levels; an increase that is comparable to that simulated in response to radiatively forced climate change (11+/-6 per cent). Assessments of the effect of increasing carbon dioxide concentrations on the hydrological cycle that only consider radiative forcing will therefore tend to underestimate future increases in runoff and overestimate decreases. This suggests that freshwater resources may be less limited than previously assumed under scenarios of future global warming, although there is still an increased risk of drought. Moreover, our results highlight that the practice of assessing the climate-forcing potential of all greenhouse gases in terms of their radiative forcing potential relative to carbon dioxide does not accurately reflect the relative effects of different greenhouse gases on freshwater resources.
H34D-07
Global Trends in Potential and Actual Evapotranspiration Based on 20 Years of Satellite Observations
Evapotranspiration (ET) or the flux of water to the atmosphere from soil evaporation and plant transpiration plays a central role in the global water cycle and both responds to and influences the evaporative demand of the atmosphere. Recent analyses suggest a decrease in pan evaporation as a result of reduced energy available for evaporation due to solar dimming. Our objectives are to 1) quantify temporal changes in both potential and actual ET for different regions of the globe and 2) determine if this change is consistent with reported decreases in pan evaporation. We first describe a modified Priestley-Taylor model of evaporation that we have recently developed and tested for application with satellite remote sensing. We apply the model using 20 years of satellite-based observations from the Advanced Very High Resolution Radiometer (AVHRR) along with additional satellite-based estimates of net radiation from the surface radiation budget (SRB) experiment. Together, these data provide sufficient information on surface and near-surface biophysical properties (visible and near-infrared reflectance, surface temperature, atmospheric humidity, net radiation) to estimate the both actual and potential ET. We validate the method by comparing predictions against eddy covariance measurements of latent energy flux at 16 sites from around the globe with contrasting vegetation and meteorological conditions. Temporal trends and spatial patterns in potential and actual ET will be discussed in the context of decreased pan evaporation and potential feedbacks between evaporative demand, surface energy balance and plant and soil evaporation.
H34D-08
Can the worldwide changes in evaporative demand be reconciled with changes expected due to global warming?
Evaporation pans are devices, often used by agricultural scientists and engineers to schedule irrigation. They are also widely used by ecologists who recognise the ratio of ‘actual to potential' as being of fundamental significance. Because of the practical importance, many organisations, in many countries, maintain standardised networks of pan evaporimeters. Using those data, the scientific community now has a reasonably clear picture of worldwide changes in evaporative demand over the last 30 to 50 years. We are also starting to gain a picture of how the observed changes in pan evaporation are related to changes in radiation, humidity and wind, as well as with changes in actual evapotranspiration. In this paper we attempt to reconcile the observed trends in pan evaporation observations with (1) generic expectations for evaporative demand in a warmer world, (2) GCM model output and (3) oceanic observations of changing evaporation and rainfall.