Abstract
Agricultural drought has traditionally been monitored using indices that are based on above-ground measures of temperature and precipitation that have lengthy historical records. However, the period-of-record length for soil moisture networks is becoming sufficient enough to standardize and evaluate soil moisture anomalies and percentiles that are spatially and temporally independent of local soil type, topography, and climatology. To explore these standardized measures in the context of drought, the U.S. Climate Reference Network hourly standardized soil moisture anomalies and percentiles were evaluated against changes in the U.S. Drought Monitor (USDM) status, with a focus on onset, worsening, and improving drought conditions. The purpose of this study was to explore time scales (i.e., 1–6 weeks) and soil moisture at individual (i.e., 5, 10, 20, 50, and 100 cm) and aggregated layer (i.e., top and column) depths to determine those that were more closely align with evolving drought conditions. Results indicated that the upper-level depths (5, 10, and 20 cm, and top layer aggregate) and shorter averaging periods were more responsive to changes in USDM drought status. This was particularly evident during the initial and latter stages of drought when USDM status changes were thought to be more aligned with soil moisture conditions. This result indicates that standardized measures of soil moisture can be useful in drought monitoring and forecasting applications during these critical stages of drought formation and amelioration.
Significance Statement
Drought is normally monitored by making inferences from temperature and precipitation observations. In this study, we explored whether soil moisture data would improve our ability to monitor evolving drought conditions. Results showed that soil moisture observations were drier than usual prior to U.S. Drought Monitor onset for nearly 80% of events and worsening drought weeks. For improving weeks, soil moisture observations were only slightly drier than usual or near normal. This was more pronounced in the initial and final few weeks of drought. This suggests that applications of soil moisture measurements to monitor and anticipate evolving drought conditions are best focused on the critical stages of drought formation and termination.
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