The aim of this research was to evaluate the impact of regional change in ET on groundwater level changes and the assessment and development of a groundwater sustainability index for climatically diverse regions acrossNebraska during 2000-2014. Irrigation in the selected regions is predominantly supplied by groundwater.
The hypothesis is that groundwater use can become sustainable if the regional evapotranspiration (ET) is managed so that it equals the ET of vegetation that is native to the region. Site locations were Box Butte, Chase, Dundy, Holt LNNRD and York Counties and 3 ecosystems were evaluated: native vegetation, dryland and irrigated cropping systems. Methodology consisted of remote sensing data analysis, GIS modeling, ASCE-Penman Monteith Evapotranspiration equation (Allen et al., 2005) and analysis of historical data.
Precipitation, ET, NDVI, Kc and ET weighed to obtain county average ET .Groundwater level declines and groundwater storage data were assessed and compared with ET. Also the same procedures were applied at the township scale for 5 townships in each county. Results showed that precipitation increased from west to east ranging between 406-693 mm.y-1. York County had the overall highest ET values, and Box Butte County had the lowest.
Annual native vegetation ET ranged 382-644 mm.y-1 and county annual ET ranged 415-720 mm.y-1 from west to east. The highest groundwater level declines were in Box Butte, Dundy and York Counties (0.26 m.y-1) and the lowest in Holt LNNRD (0.12 m.y-1).Sustainability index (SI) indicated that Holt LNNRD was the most sustainable in water use (SI=0.94) and Chase County the least sustainable (SI=0.88). County average ET exceeded native vegetation ET in all cases due to the higher ET of irrigated crops.
The positive correlation between ET increases and reduction in water storage showed a significant correlation (0.62) at p-value < 0.05 level confidence, n=10. Higher SI resulted in lower reduction in groundwater storage. The negative correlation between SI and reduction in groundwater storage (-0.69) proved to be significant at p-value< 0.025, n=10. However, data suggested that reducing county average ET, approaching that of native vegetation, which kept water levels in balance in the past, could be a promising practice.
Source: University of Nebraska Lincoln
Authors: Maria A. Mulet Jalil