Quantifying the Relative Roles of Progressive Land Use Change, Irrigation, and Remote Forcing in Southern Great Plains Precipitation Variability

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

This project seeks to understand the causes of precipitation variability in the US Southern Great Plains (SGP), roughly equivalent to the region covered by Nebraska, Missouri, Oklahoma, Arkansas, Texas, and Louisiana. This area is among the most drought-prone in the US, with substantial economic impacts including the severe consequences of the Dust Bowl and losses in the tens of billions of dollars in 2011 and 2012. Precipitation anomalies in the region, including rainfall deficits contributing to droughts, are generally attributed to changes in the large-scale atmospheric circulation brought about by fluctuations in sea surface temperature (SST). But local land surface conditions can act to intensify remotely-forced precipitation variability through several mechanisms. For instance, lack of rainfall leads to dry soil and reduced evaporation, which reduces the moisture source available for precipitation and causes further rainfall reduction. Also, warm dry conditions due to drought promote ridging in the mid-troposphere, which can further reduce precipitation as rain-bearing systems move around the ridge. This project considers the extent to which the strength of such land surface feedbacks to precipitation has changed due to changes in land use and land cover change in the SGP. Since the 1950s much of the native vegetation in the region has been replaced by agricultural crops, and this substitution can affect the exchange of water between the surface and the overlying atmosphere. Native plants have much deeper roots and can access soil moisture from reserves unavailable to crops, thereby maintaining the source of moisture for precipitation even during dry spells. Also, crops tend to green up earlier than native plants, so that plant uptake and transpiration of soil moisture begins earlier on agricultural land. The enhanced early season moisture demand could increase drought susceptibility later in the growing season. A further consideration is the widespread adoption of irrigation, which can increase evaporation and transpiration during wet years but is not reliable during exceptional drought years. The research is conducted through a suite of simulations performed using the Weather Research and Forecasting (WRF) model, a regional model which is configured with high resolution over the great plains. WRF is coupled to the Community Land Model (CLM), in which land cover can be specified to represent past and present vegetation cover and varying levels of irrigation can be represented. Experiments consist largely of WRF-CLM simulations over the period of the North American Regional Reanalysis (NARR, 1979-2012), first with land cover held fixed to the modern land cover attributes (from the National Land Cover Database). This simulation is then compared with additional simulations in which land cover is configured to represent conditions during the pre-settlement era (based on data representing the year 1850), intermediate land use conditions around 1920, and the dust bowl. Simulations for modern and intermediate land use are performed with and without irrigation. The work has societal broader impacts due to the economic and societal impact of drought in the SGP. The findings of the research may be useful to the agricultural sector and SGP water managers as they make decisions regarding changes in land and water use in the region. In addition to publications in peer-reviewed research journals, results of the research will be distributed through the University of Nebraska-Lincoln Extension and other extension services across the SGP. In addition the project will support and train two graduate student, thereby providing for the future workforce in this research area.