Evaluation of Soil Moisture-Based Indices for Quantifying Agricultural Drought in the Southern Great Plains.

Agricultural drought is characterized by low soil moisture levels that negatively affect agricultural production, but because sufficient data have historically been lacking, measured soil moisture is not incorporated into any of the drought indices widely used to define agricultural drought. As high quality in-situ soil moisture monitoring networks are becoming more widespread, soil moisture-based indices are a logical choice for monitoring agricultural drought, but potential indices and their links to agricultural production remain understudied. Our objective was to use soil moisture data from the densely monitored Oklahoma and West Texas Mesonet systems to develop readily understandable, scientifically sound, soil moisture-based drought indices to quantify agricultural drought. We identified promising indices by quantifying their relationships with county-level crop yields for hay and the primary warm-season (cotton) and cool-season crops (wheat) in Oklahoma and West Texas. Candidate indices ranged from the fundamental soil matric potential (MP), to soil water content (SWC), to fraction of available water capacity (FAW), which incorporates soil physical property information in its calculation. We found that for each drought index, the period during the year when crop yields were most highly correlated with soil moisture varied by crop, with drought during early-spring corresponding with reduced wheat yield and early-summer drought corresponding with decreased hay yield. When evaluated across counties, the soil moisture-yield relationships were stronger for FAW than SWC for both wheat and hay, indicating that soil physical property information may be an important component soil-moisture based drought index calculation. However, when drought indices were assessed as anomalies, FAW offered no advantage over MP or SWC. We therefore conclude that in the absence of soil physical property information, calculating soil moisture based drought indices as anomalies is preferable those based on raw values.