This paper investigates whether remote sensing evapotranspiration estimates can be integrated by means of data assimilation into a distributed hydrological model for improving the predictions of spatial water distribution over a large river basin with an area of 317,800 km2. A series of available MODIS satellite images over the Haihe River basin in China are used for the year 2005. Evapotranspiration is retrieved from these 1×1 km resolution images using the SEBS (Surface Energy Balance System) algorithm. The physically-based distributed model WEP-L (Water and Energy transfer Process in Large river basins) is used to compute the water balance of the Haihe River basin in the same year. Comparison between model-derived and remote sensing retrieval basin-averaged evapotranspiration estimates shows a good piecewise linear relationship, but their spatial distribution within the Haihe basin is different. The remote sensing derived evapotranspiration shows variability at finer scales. An extended Kalman filter (EKF) data assimilation algorithm, suitable for non-linear problems, is used. Assimilation results indicate that remote sensing observations have a potentially important role in providing spatial information to the assimilation system for the spatially optical hydrological parameterization of the model. This is especially important for large basins, such as the Haihe River basin in this study. Combining and integrating the capabilities of and information from model simulation and remote sensing techniques may provide the best spatial and temporal characteristics for hydrological states/fluxes, and would be both appealing and necessary for improving our knowledge of fundamental hydrological processes and for addressing important water resource management problems.

A distributed model for simulating the land surface hydrological processes in the Heihe river basin was developed and validated on the basis of considering the physical mechanism of hydrological cycle and the artificial system of water utilization in the basin. Modeling approach of every component process was introduced from 2 aspects, i.e., water cycle and energy cycle. The hydrological processes include evapotranspiration, infiltration, runoff, groundwater flow, interaction between groundwater and river water, overland flow, river flow and artificial cycle processes of water utilization. A simulation of 21 years from 1982 to 2002 was carried out after obtaining various input data and model parameters. The model was validated for both the simulation of monthly discharge process and that of daily discharge process. Water budgets and spatial and temporal variations of hydrological cycle components as well as energy cycle components in the upper and middle reach Heihe basin (36 728 km²) were studied by using the distributed hydrological model. In addition, the model was further used to predict the water budgets under the future land surface change scenarios in the basin. The modeling results show: (1) in the upper reach watershed, the annual average evapotranspiration and runoff account for 63% and 37% of the annual precipitation, respectively, the snow melting runoff accounts for 19% of the total runoff and 41% of the direct runoff, and the groundwater storage has no obvious change; (2) in the middle reach basin, the annual average evapotranspiration is 52 mm more than the local annual precipitation, and the groundwater storage is of an obvious declining trend because of irrigation water consumption; (3) for the scenario of conservation forest construction in the upper reach basin, although the evapotranspiration from interception may increase, the soil evaporation may reduce at the same time, therefore the total evapotranspiration may not increase obviously; the measure of changing the farmland to pasture land in the middle reach basin has obvious effects on decreasing evapotranspiration, increasing the discharge at Zhengyixia, and decreasing the storage deficit; reducing the irrigation surface water use in the middle reach basin has obvious functions on increasing the discharge to downstream but the groundwater exploitation increasing should be restricted to prevent the groundwater table decline.

The water and energy transfer process (WEP) model, which has been successfully applied to urbanized watersheds, is improved for hydrologic modeling in an agricultural watershed by developing a paddy model. The new model is used to simulate the Yata watershed at 100 m grid size and variable time step. A 4 year 3 month simulation is performed for model calibration and verification. Model results are compared to measured values of river discharges, groundwater levels, and soil moisture contents. Sensitivity of annual water budgets and hourly river discharges to rainfall estimation error is also analyzed. The verified model is applied to simulate the long-term variation of water budgets and flood-retarding effects of paddy fields in the watershed. Study results indicate that: (1) the WEP model is applicable to agricultural watersheds in addition to urbanized ones; (2) annual weather variations give more obvious impact on water budgets than urbanization in the agricultural watershed; and (3) paddy fields play ...

Many soil and water conservation (SWC) measures have been applied in the Jinghe River Basin to decrease soil erosion and restore degraded vegetation cover. Analysis of historical streamflow records suggests that SWC measures may have led to declines in streamflow, although climate and human water use may have contributed to observed changes. This paper presents an application of a watershed-scale, physically-based eco-hydrological model—the Regional Hydro-Ecological Simulation System (RHESSys)—in the Jinghe River Basin to study the impacts of SWC measures on streamflow. Several extensions to the watershed-scale RHESSys model were made in this paper to support the model application at larger scales (>10,000 km2) of the Loess Plateau. The extensions include the implementation of in-stream routing, reservoir sub-models and representation of soil and water construction engineering (SWCE). Field observation data, literature values and remote sensing data were used to calibrate and verify the model parameters. Three scenarios were simulated and the results were compared to quantify both vegetation recovery and SWCE impacts on streamflow. Three scenarios respectively represent no SWC, vegetation recovery only and both vegetation recovery and SWCE. The model results demonstrate that the SWC decreased annual streamflow by 8% (0.1 billion m3), with the largest decrease occurring in the 2000s. Model estimates also suggest that SWCE has greater impacts than vegetation recovery. Our study provides a useful tool for SWC planning and management in this region.