Using satellite imagery to compare land cover and water resources in two counties of the Nebraska Sandhills

The Nebraska Sandhills comprise the most intact grassland habitat in the world and 95% of land use consists of low intensity cattle grazing. Water is a key resource for cattle and for growing hay forage in this semi-arid grassland. Ranchers rely on either naturally occurring wet meadows or center pivot irrigation systems (CPIS) to produce hay. With the possibility of climate change creating more frequent extreme weather events, more flooding events or severe droughts could affect land and water resources in the Sandhills. With potentially more wet/dry extremes in the future, an understanding of the way water resources respond, and the different strategies of landowners, will be important in assessing the overall resilience of the Nebraska Sandhills in the face of a changing climate. We compared two adjacent, similarly sized, counties, Grant and Hooker, in the central Sandhills that differ in the amount of naturally occurring, ground water-fed meadows during the period 2002-2019, spanning wet and dry years. ArcGIS and the USDA’s Cropland Data Layer (known as “CropScape”), an annual, satellite imagery-derived, land cover map, were used to quantify overall landcover, especially the cover of wet meadows and the number of CPIS. In 2016, an average-to-moderate precipitation year, Grant County had approximately 10 times more open water area and 9 times more wet meadow area than Hooker County. In contrast, Hooker County had 19 times more barren ground area and nearly twice as many CPIS as Grant County. Furthermore, in drought years, the amount of barren ground increased in both counties, nearly doubling in Hooker County in 2006, a year when annual precipitation was only 66% of normal precipitation rates. Drought also increased the acreage devoted to irrigated crops, particularly in 2006 in both counties, with nearly two times as many acres in Grant County and nearly five times as many acres in Hooker County. Additionally, open water acreage decreased by nearly 50% following the 2006 drought and wetland cover types increased. This analysis showed that different Sandhill counties have contrasting water resources, with an abundance of naturally occurring wet meadows in Grant County, whereas Hooker County has fewer wet meadows and more CPIS. Furthermore, our analysis suggests that the landscape changes during drought years, with a decline in wet meadows, an increase in barren ground and an increase in CPIS and other irrigated systems for crops. In a changing climate, with potentially more extremes in precipitation, the diversity of strategies exemplified by these two counties will be important to inform adaptative responses. Overall, this research will contribute to a better understanding of the sustainability of land use and the future of groundwater resources in the Nebraska Sandhills.