Water balance approach has been applied to wide spectrum of the hydrological and water resources studies including climate change, evapotranspiration, soil moisture, runoff, fluvial transport and irrigation management. In an agriculture based economy such as state of Nebraska (NE), water allocation for crop water irrigation management is a crucial issue. Knowledge of soil water availability will be very useful for various water management issues in NE where the long-term viability of water supplies in the state is threatened by several consecutive years of drought and over-pumping of groundwater supply from Ogallala aquifer. Soil water content is the most variable of all the resources in the high plains (Robinson nd Hubbard, 1990). Availability of water in the root zone is affected by various factors including soil properties, climate variables, crop phenology, topography and groundwater table depth etc. A GIS based daily soil water balance model was developed to simulate daily soil water content in the root zone (SWC). The model continuously simulates spatial distribution of each water balance component such as evapotranspiration, runoff and precipitation and irrigation on a daily time step. The entire model was implemented in ArcGIS using Visual Basic Applications and ArcObjects. Model was tested at three different sites (two irrigated and one dryland) with corn and soybean crops for year 2001 to 2003 growing season near Mead, NE. Statistical measures confirmed satisfactory model performance for all validation cases (r2 ranging from 0.32 to 0.95). Model estimation soil water content in the root zone was better under the dryland conditions compared to irrigated condition. Soil water balance model was used to estimate the status of SWC.
