Economic cost of augmenting streamflows in the Republican River in Southwest Nebraska

There has been considerable discussion about the impact of groundwater irrigation development on streamflows in the Republican River in Southwest Nebraska. Groundwater levels in Southwest Nebraska have declined up to 30 feet or more and streamflows at the same gauges have declined by nearly two-thirds. Linear programming and simulation models were used to determine the least cost of reducing the impact of groundwater irrigation on streamflows in the Republican River. The groundwater water was constrained through successive reductions of both gross and consumed irrigation water. The yield-gross water response function is curvilinear while that with consumed water is linear. Hence in the latter case, land is either irrigated or goes under dryland production. When gross water is restricted, the optimum response was initially to keep all irrigated land in production but with a slight reduction in applied water and output. At higher restrictions, it becomes economic to shift irrigated land to dryland. In contrast, when consumed water is restricted, the optimal response is to reduce irrigated acres, rather than reduce applied water. The first land to shift to dryland in both scenarios is gravity on sandy soils. Continuous corn is the best irrigated crop under both scenarios, while the best dryland alternatives are wheat-corn ecofallow for silt soils and continuous grain sorghum for both the loam and sandy soils. Also, when costs are defined as the loss in net returns, the average cost per acre foot of reducing consumed water ranged from a high of $189.34 at a 20 percent reduction in gross application to a low of only \$132.69 at 80 percent reduction. On the other hand, if consumed water is restricted rather than gross water, the average cost per acre foot of reduced consumptive use is from a low of $109.93 to a high of only \$129.37 with the same associated percent reduction of gross water. Thus it is more cost effective to restrict consumed water directly than restricting gross water applications