Impact of Irrigated Crop Production on Groundwater Level Dynamics in Nebraska.

Despite the general perception that the relationship between irrigation withdrawals and change in groundwater is well understood, there is a dearth of published papers exploring this link with field evidence.  Additionally, the few papers on the impact of irrigation on groundwater have dealt in limited geographic and temporal scales, and have been severely constrained by a lack of in-field irrigation data. A new database collected over 9 years from thousands of corn and soybean fields in Nebraska contains extensive, producer-reported data from flow meters, including annual irrigation application amounts and yields for individual fields. This high quality database presents an opportunity to analyze spatial and temporal distributions of annual irrigation groundwater withdrawals, as well as to investigate the relationship between irrigation and groundwater dynamics in two distinct agricultural regions of Nebraska. Spatial distribution of irrigation amounts in Nebraska are largely explained by soil type, irrigation system, and both precipitation and evapotranspiration patterns. Soil type in particular appears to be a defining factor in terms of predicting irrigation usage spatially.  With equal precipitation and evapotranspiration amounts, the region with heterogeneous, sandy soil receives twice as much irrigation compared to the more homogeneous, loamy region – indicating that commonly used simple models accounting for weather factors and crop type alone will not produce accurate irrigation predictions for two distinct regions. Groundwater data is currently being analyzed and preliminary findings show that large spring-to-fall declines in groundwater levels (averaging 1.5 meters in some years) are attributable to significant seasonal irrigation withdrawals.