Soil Moisture Observations as Key Indicators of Warm Season Drought Onset and Recovery
The state of soil moisture at a location is a critical indicator of the potential for successful vegetation growth, runoff rates and soil erosion, and regional hydrology. Because of the difficulties of collecting accurate soil moisture observations, and the relatively coarse distribution of those stations and networks in existence, soil moisture model outputs and subjective agricultural reports of soil state are more commonly used in drought monitoring. However, the distribution of accurate soil moisture measurements has improved recently in the United States with the deployment of soil moisture and temperature probes by the U.S. Climate Reference Network (USCRN) Program. While still not as numerous as would be required by a purpose-designed network for drought monitoring, the USCRN use of redundant measurements at standard depth levels provides high quality references that can be used to understand the relationships between drought as determined by multiple indicators and by soil moisture alone. The USDA Soil Climate Analysis Network (SCAN), while lacking redundancy, does have a key advantage of longer time series for the calculation of normals and departures from normals. Both data sets will be used to examine the drought-soil moisture relationship, with the state of drought determined by the U.S. Drought Monitor (USDM).
Soil moisture measurements from longer term SCAN records will be examined in lead/lag relationship to a variety of temperature and precipitation indicators, including location specific Standardized Precipitation Index (SPI) values and Standardized Precipitation-Evapotranspiration Index (SPEI) values. A closer examination of the recent evolution of drought in the U.S. will be conducted with both SCAN and USCRN soil moisture observations. For example, in northern Arkansas, it was clearly determined that USCRN daily soil moisture changes were a leading indicator of USDM drought status changes, with a brief lag between soil moisture reduction and the recognition of drought, but a longer delay between soil moisture recovery and drought status reduction. With SCAN, a sufficient length of time series allowed for the calculation of monthly soil moisture departures from normal at a station in the same area, and this graph clearly shows the period with the most below normal soil moisture (red bars) occurred at the beginning of drought as determined by USDM records (black bars), and soil moisture was largely recovered shortly after the peak of the drought which carried on for many more months. The utility of USCRN soil moisture and temperature data, like SCAN before it, is quite substantial in monitoring drought in the United States, and this presentation will support the further use of in situ soil moisture observations in drought monitoring.