Defining the Multiscalar Index Timescale—Soil Water Depth Continuum for the Southwestern United States

Abstract

The lack of long-term reliable in-situ soil moisture data sets creates unique drought monitoring challenges for the semi-arid Southwestern United States (“Southwest”). Land managers use multiscalar meteorological drought indices, like the Standardized Precipitation Index (SPI) and Standardized Precipitation-Evapotranspiration Index (SPEI), as proxies for soil water availability; however, objectively identifying the index and timescale that best represents soil water availability in semi-arid environments remains a significant gap for applying available climate information to land management action. Here, we couple site-specific soil modeling with high resolution, spatially continuous meteorological data sets to define the relationship between index timescale length and soil water availability of different depths. By creating new matric potential indices (MPI) from 0 to 200 cm, we correlate MPI time series with the SPI and SPEI at different timescale lengths to identify the best index-timescale combination for each depth. Results indicate the general relationship between highest correlating index-timescale and MPI-depth operates roughly on a 1-month:5 cm step progression at shallow depths (<80 cm). Analysis by soil texture class shows that soils with higher clay content produce shallower sloped relationships (>1-month:5 cm) than sandy soils (<1-month:5 cm). Overall, the SPI produced higher correlations and less error with the MPI compared to the SPEI across all texture classes and depths. Thus, the SPI is a good indicator of soil water availability at shallow depths (<80 cm) and should be considered for soil drought monitoring on Southwestern drylands. However, land managers should consult local soils information, if available, given the impacts of texture class on timescale-depth relationships. © 2023. The Authors.