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ARS Home » Northeast Area » University Park, Pennsylvania » Pasture Systems & Watershed Management Research » Research » Publications at this Location » Publication #106460
Title: ON EVALUATING THE SPATIAL DISTRIBUTION OF WATER BALANCE IN A SMALL WATERSHED, PENNSYLVANIA

Author
item YU, ZHONGBO - PENN STATE UNIVERSITY
item Gburek, William
item SCHWARTZ, F - OHIO STATE UNIVERSITY

Submitted to: Hydrologic Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/1999
Publication Date: 1/5/2000
Citation: Yu, Z., Gburek, W.J., Schwartz, F.W. 2000. Evaluating the spatial distribution of water balance in a small watershed, Pennsylvania. Hydrologic Processes. 14:941-956.

Interpretive Summary: Models developed to represent watershed response over large geographic areas are typically simplified by using average values of hydrologic parameters and are generally applied at a single point in order to limit the time necessary to run the model on the computer. We have shown that a complex model originally developed for a point application to simulate the water balance, including evapotranspiration, soil water, runoff, and drainage below the root zone, can be modified for evaluating the spatial variability of the water balance over a watershed using daily precipitation, temperature, and other hydrologic variables, while still requiring only limited computer time. To demonstrate the model's abilities, we compared simulations of daily soil water values to observed data from a small agricultural watershed in Pennsylvania, as well as to results from a model that simulates soil water using a theoretical soil water flow equation. The influences of soil properties and land use on th daily water balance were then evaluated using our model. In general, the simulated distribution of hydrologic components showed the combined effect of soil and land use and compared well with observed data. For identical meteorological conditions and land use, evapotranspiration and surface runoff increase as the available water capacity of soil increases, while drainage from the root zone decreases. Among the land uses considered, alfalfa produced the highest values of soil moisture deficit and evapotranspiration, with lower surface runoff and subsurface drainage, while soybeans showed the opposite trend. The intuitive and simple model developed provides a valuable tool for the management of water resource problems in small watersheds.

Technical Abstract: A conceptual water-balance model was modified from a point application to be distributed for evaluating the spatial distribution of watershed water balance based on daily precipitation, temperature, and other hydrologic parameters. The model was calibrated by comparing simulated daily variation in soil moisture with field observed data and results of another model that simulates the vertical soil moisture flow by numerically solvin Richards' equation. The impacts of soil and land use on the hydrologic components of the water balance, such as evapotranspiration, soil moisture deficit, runoff, and subsurface drainage, were evaluated with the calibrated model in this study. Given the same meteorological conditions and land use, the soil moisture deficit, evapotranspiration, and surface runoff increase, and subsurface drainage decreases, as the available water capacity of soil increases. Among various land uses, alfalfa produced high hsoil moisture deficit and evapotranspiration and lower surface runoff and subsurface drainage, while soybeans produced an opposite trend. The simulated distribution of various hydrologic components shows the combined effect of soil and land use. Simulated hydrologic components compare well with observed data. The study demonstrated that the distributed water balance approach is efficient and has advantages over the use of single average value of hydrologic variables and the application at a single point in the traditional practice.