Future efforts in large scale hydrology will concentrate on
the integration of hydrologic models with climate simulations.
Some recent field studies have been designed to
address this objective, although others cannot fully satisfy
the requirements of collecting longer time scale data for
monitoring the hydrological cycle on a seasonal basis.
Nevertheless, in these experiments the energy exchanges
between the land surface and atmosphere from the local
(i.e., on the order of 10
km) to the regional scale (i.e.,
on the order of 10
km) were observed. This information is
essential in scaling up fluxes and processes between the land
surface and atmosphere, and thus move closer towards
integrating hydrological processes in climate models. A
brief discription of some these experiments together with
how they may contribute towards advancing hydrologic
research is given below.
The ECHIVAL Field Experiment in a Desertification-threatened
Area (EFEDA) was conducted during the summer
of 1991 in the Castilla-Monch region in Spain (Bolle et al.,
1993). Measurements included aircraft- and satellite-based
remote sensing observations and aircraft- and ground-based
flux measurements. Three sites separated by 70 km were
well instrumented representing areas on the order of 30 km
.
The observations will permit the representation of land
surface processes over an area about 10
km
, which is
essentially the size of a GCM grid box. There are plans to
use the French Weather Service meso-beta-scale model to
investigate land-surface-atmosphere interactions and to
develop methods of integrating satellite remote sensing data.
These data have the potential to specify surface properties
such as vegetation cover and surface resistance over large
areas (Noilhan et al., 1991a).
The HAPEX-Sahel experiment was conducted in Niger,
West Africa over a two year period 1991-1992 (Goutorbe et
al., 1994). This relatively long term study will allow the
investigation of hydrological processes such as soil moisture
profile in the vadose zone, precipitation and runoff operating
at the seasonal and even annual time scales. Ground,
aircraft and satellite measurements were made in an attempt
to observe land-surface and atmospheric properties, processes
and fluxes from local to the GCM grid scale. Three sites
were selected along the synoptic precipitation and vegetation
gradient existing in the region. These sites contained
mesurements that reflected the impact on ABL flow, which
are assumed to reflect upwind conditions over length scales
on the order of 10
km. Similar to EFEDA, a mesoscale
model (e.g., Bougeault et al., 1991) will be used to evaluate
the effects of the land surface cover on area-integrated
energy fluxes. The utility of remote sensing data to provide
information on spatial variability of land-surface characteristics
will be assessed, as well as methods to integrate this
information into atmospheric and hydrologic models.
The hydrology of this region is unique, and in general the
watersheds cannot be modeled using classical hydrological
theory developed in more humid climates. Consequently,
new and innovative methods will be required. Besides the
highly variable rainfall in both space and time (see earlier
discussion), the Sahelian region is mainly the hydrology of
endoric (inward draining) basins whose sizes are generally
on the order of 10
km
. Within these areas, ponds are
created during the rainy season and eventually dry out
several months after the rain stops. Other basins which have
outflow generally have little runoff because of the lack of
topography. Therefore, solving evaporation as a residual in
the water balance over large areas will be very difficult.
Instead, the use of micrometeorological techniques for local
evaporation estimates and approaches using ABL data for
regional estimates will be utilized. This will facilitate
interaction between the hydrological and atmospheric
modelers.
Another long term study for understanding the regional water cycle was conducted in the Hieche River basin in China. The Hieche River Basin Field Experiment (HEIFE) was conducted over a two year period, 1990-1992 (Jiemin et al., 1993). Shorter term intensive observation periods during the two year period included energy flux measurements, soil moisture and atmospheric boundary layer measurements. Continuous measurements of the surface energy fluxes and meteorological conditions over the two year period were made at five sites. Also seasonal changes in surface conditions for the region were monitored from satellites. The river basin is situated within the Gobi Desert and contains significant spatial gradients in vegetation cover and soil moisture. Some preliminary observations indicate unique atmospheric features were caused by the contrasting dry and wet regions, that is a desert-oasis effect at large spatial scales. Sand dust was shown to play a dramatic role in the radiation balance of the desert versus the oasis. In addition, the planetary boundary layer had two distinct regions. The lower layer was seemingly controlled by local conditions while the upper one was mainly affected by regional circulations. How these surface-atmospheric interactions will affect the longer term hydrologic cycle will have to be investigated.
The Boreal Ecosystem-Atmosphere Study (BOREAS) has similar objectives to FIFE, except the project will have a major focus on the biome's role in carbon cycling. In addition, the role of snowmelt on the hydrologic cycle and fate of biogeochemical fluxes will be explored. Observations collected through the spring and summer of 1994 included the transport of momentum, energy and trace gas fluxes between the boreal forest ecosystem and the atmosphere. These fluxes were measured by ground- and aircraft-based sensors. Remote sensing data from aircraft and satellite systems will be an integral part of this project, similar to FIFE, EFEDA, and HAPEX-Sahel. The plan is to use the remote sensing data as means to extend local land surface and atmospheric processes to regional scales (Dozier, 1992).
The Convection and Precipitation/Electrification (CaPE)
Experiment was a multi-agency field program conducted in
central Florida in the summer of 1991. The focus of the
experiment was to study the development of mesoscale
meteorological conditions leading to convection initiation,
downbursts, and tornadoes, and to develop improved
techniques for performing short period forecasts of these
events (Williams et al., 1992). During this experiment, the
diverse data set that was collected spawned additional
research projects, including the CaPE Hydrometeorology
Project (CHymP; C. Laymon, personal communication,
1994). The objective of CHymP is to develop and apply a
method to diagnose large-scale (namely on the order of
25,000 km
) land and atmosphere water budget components.
The underlying philosophy is that these techniques can be
tested and applied over regional-scale areas in conjunction
with GCIP program. Data used in ChymP were obtained
from many sources including supervised field measurements,
unsupervised temporary and permanent gaging stations,
surface radars, radiosondes, aircraft-based instruments, and
aircraft and satellite remote sensing systems. A model of
land surface-atmosphere water and energy exchange has
been developed representing a new contribution to the study
of large-scale hydrologic processes and parameterizations.
This model was used in studies of the sensitivity of surface
fluxes and runoff to soil and landcover characterization.
Preliminary results from CHymP are raising many questions
concerning the treatment of scale-dependence of land
surface-atmosphere interactions on spatial and temporal
variability (Laymon et al., 1994).