Modeling Crop Water Demand and Root Zone Flow Processes at Regional Scales in the Context of Integrated Hydrology
California Department of Water Resources
In developed watersheds, the stresses on surface and subsurface water resources are generally created by groundwater pumping and stream flow diversions to satisfy agricultural and urban water requirements. The application of pumping and diversion to meet these requirements also affects the surface and subsurface water system through recharge of the aquifer and surface runoff back into the streams. The agricultural crop water requirement is a function of climate, soil and land surface physical properties as well as land use management practices which are spatially distributed and evolve in time. In almost all modeling studies pumping and diversions are specified as predefined stresses and are not included in the simulation as an integral and dynamic component of the hydrologic cycle that depends on other hydrologic components as well as anthropogenic influences. To address this issue, California Department of Water Resources has developed a new root zone module that can either be used as a stand-alone modeling tool or can be linked to integrated hydrologic models. The tool, named Integrated Water Flow Model Demand Calculator (IDC), computes crop and urban water requirements under user-specified climatic, land-use and irrigation management settings at regional scales, and routes the precipitation and irrigation water through the root zone using physically-based methods. In calculating the crop water requirement, IDC uses an irrigation-scheduling type approach where irrigation is triggered when the soil moisture falls below a user-specified level. Water demands for managed wetlands, urban areas, and agricultural crops including rice, can either be computed by IDC or specified by the user. For areas covered with native vegetation water demand is not computed, irrigation is set to zero and only precipitation is routed through the root zone. Many water management practices such as deficit irrigation, re-use of irrigation return flow, flooding and draining of rice fields and seasonal wetlands are addressed. IDC operates on a finite-difference or finite-element computational grid even though it does not use finite-element or finite-difference simulation techniques. The utilization of a computational grid facilitates accurate representation of spatially distributed input data as well as easy linking to integrated hydrologic models that use such grids. When used as a stand-alone tool, IDC assumes that the irrigation amount equals the computed or specified water demand. When linked to integrated hydrologic models, the irrigation amount equals the sum of simulated pumping and diversions which may be less than the IDC-computed water demand based on available aquifer storage and stream flows. In such cases IDC effectively computes increased water demands for the next time step as well as decreased groundwater recharge and surface runoff. IDC has been tested by applying it to three counties in California; the results in terms of applied water demand, evapotranspiration of applied water (ETAW) and effective precipitation compared well to data that are available through 1998 to 2001.