Mapping Spatially Interpolated Precipitation, Reference Evapotranspiration, Actual Crop Evapotranspiration, and Net Irrigation Requirements in Nebraska: Part II. Actual Crop Evapotranspiration and Net Irrigation Requirements

Abstract

With the world population expected to increase by 3.7 billion people by 2050, it is anticipated that the additional food and fiber required to feed future generations will impose greater pressure on freshwater resources. Approximately 70% to 80% of global freshwater withdrawal is for irrigation, and more than 40% of food is produced on irrigated land, which comprises only 18% of the total land allocated to food and fiber production. To evaluate future water supply and demand in relation to food production, it is necessary to quantify actual crop evapotranspiration (ETa) and irrigation water requirements (NIR) of various crop production systems. Long-term (1986-2009) growing season (1 May to 30 September) ETa and NIR for maize and soybean were quantified for all 93 counties in Nebraska. There was a gradual decrease in ETa totals from the western part (zone 1) of the state toward the eastern part (zone 4). For maize, the growing season total ETa ranged from 801 mm in Cheyenne County (zone 1) to 484 mm in Butler County (zone 4), with a statewide average of 634 mm. For soybean, the growing season ETa ranged from 705 mm in Cheyenne County to 430 mm in Butler County, with a statewide average of 559 mm. On a statewide average, maize ETa was 75 mm (12%) higher than soybean ETa. There was a gradual decrease in NIR totals from the western part of the state toward the eastern part. The NIR for maize ranged from 575 mm in the western part to 150 mm in eastern Nebraska, with a statewide average of 289 mm. Soybean NIR ranged from 467 mm in the western part to 111 mm in the eastern counties, with a statewide average of 227 mm. On average, the NIR was greater in zone 1 than in zone 4 by 319 mm (61.7%) for maize and 267 mm (63.04%) for soybean. July was the most critical for both crops in terms of contribution of precipitation to ETa and NIR. The statewide average July NIR was observed as 133 mm and 127 mm for maize and soybean, respectively. The statewide long-term average minimum NIR for maize was observed in 1993 as 90 mm (±88 mm) and in 1996 as 60 mm (±100 mm) for soybean, and the maximum value was obtained in 2002 as 525 mm (±188 mm) for maize and 433 mm (±173 mm) for soybean. In the last 24-year period, the statewide average maize NIR increased by 21 mm, and the average soybean NIR increased by 18 mm. The long-term average statewide NIR was 268 mm (±154 mm) for maize and 196 mm (±137 mm) for soybean. Mapping and quantification of ETa and NIR can help water users and managers, decision makers, and policymakers better allocate water resources and develop water resources management strategies for enhancing crop water productivity. These datasets and analyses can also aid in identifying locations where long-term trends in ETa, NIR, and crop productivity are decreasing so that more robust resource reallocation practices can be adopted to improve agricultural productivity.