MAPPING SPATIALLY INTERPOLATED PRECIPITATION, REFERENCE EVAPOTRANSPIRATION, ACTUAL CROP EVAPOTRANSPIRATION, AND NET IRRIGATION REQUIREMENTS IN NEBRASKA: PART I. PRECIPITATION AND REFERENCE EVAPOTRANSPIRATION

Precipitation and reference evapotranspiration are two important variables in hydrologic analyses, agricultural crop production, determining actual crop evapotranspiration and irrigation water requirements, and irrigation management. Both variables vary in space and time, and the weather networks that measure or quantify and report both variables are too sparse for practical applications by water resources planners, managers, and irrigators. Long-term (1986-2009) average annual (January to December), seasonal (growing season, May to September), and monthly (May, June, July, August, and September) precipitation and Penman-Monteith-estimated alfalfa-reference evapotranspiration (ETref) were spatially interpolated and mapped for all 93 counties in Nebraska using the spline interpolation technique in ArcGIS. Precipitation gradually increased from the western part and southwest corner (zone 1) to the eastern part (zone 4) of the state. Long-term average county annual precipitation ranged from 325 to 923 mm, with a statewide mean of 581 mm. The long-term average seasonal precipitation showed a similar trend as the annual precipitation and ranged from 215 to 601 mm, with a statewide average of 380 mm. Based on the annual average precipitation data, there was an approximately 30 mm decrease in precipitation for every 40 km from east to west. Seasonal and annual precipitation were inversely proportional to elevation with high coefficients of determination (R2 = 0.94 for annual and R2 = 0.88 for seasonal). Annual precipitation decreased between 18 and 131 mm for every 100 m increase in elevation. Seasonal precipitation decreased between 11 and 72 mm for every 100 m increase in elevation. The long-term statewide average annual ETref was 1,400 mm, with significant differences across the state: 1,662 mm (zone 1), 1,542 mm (zone 2), 1,350 mm (zone 3), and 1,285 mm (zone 4). The statewide long-term average seasonal ETref was 883 mm, with a maximum of 1,087 mm and minimum of 684 mm. The maximum monthly ETref of 268 mm was observed in July, and the minimum value of 12 mm was observed in December. The annual ETref increased by 47 mm for every 100 m increase in elevation, and the seasonal ETref increased by 29 mm for every 100 m increase in elevation. Spatially interpolated maps of precipitation and ETref can provide important background information and physical interpretation of precipitation and ETref for climate change studies in the region, which can lead to the ability to take proactive steps to balance water supply and demand through various available methods, such as changing cropping patterns to implement cropping systems with lower water demand, reduced tillage practices to minimize unbeneficial water use (soil evaporation), implementing newer drought-tolerant crop hybrids and cultivars, implementing deficit irrigation strategies, and initiating and deploying more aggressive and effective irrigation management programs.