Soil Moisture Observations and 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, 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). A close examination of the recent evolution of drought in the U.S. as indicated by the USDM will be conducted with both SCAN and USCRN soil moisture observations. For example, comparison of soil moisture anomalies and USDM status changes in northern Arkansas shows that soil moisture state is 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 amelioration. The period with below normal soil moisture occurred at the beginning of drought as determined by USDM records, and soil moisture was largely recovered shortly after the peak of the drought status 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 demonstrate the value of using large networks of in situ soil moisture observations in drought monitoring.