Exploring the Use of Standardized Soil Moisture as a Drought Indicator

Abstract

Agricultural drought has traditionally been monitored using indices based on above ground measures of temperature and precipitation that have lengthy historical records. However, the period-of-record length for soil moisture networks are 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 (USCRN) 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 timescales (i.e., 1 to 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 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.

Characterizing U.S. drought over the past 20 years using the U.S. drought monitor

“…A time series of weekly USDM drought status (Dx value) at each grid cell was generated from the gridded dataset and used to identify non‐overlapping drought events, as outlined in Leeper et al . (2021). Based on their approach, a drought event was defined as beginning on the first week the USDM status meets or exceeds moderate drought (D1) conditions and ends the last week the USDM status meets or exceeds D1, followed by three or more consecutive weeks of abnormally dry (D0) or None conditions (Figure 1).…”

Future drought propagation through the water-energy-food-ecosystem nexus – A Nordic perspective

“…In addition to those employed in this study, other standardized indices might also be appropriate to represent particular sectors. For example, the Standardized Soil Moisture Index (Leeper et al, 2021) may be used to characterize the food sector, whereas the Standardized Reservoir Supply Index (Shiau, 2003) may be helpful to represent the energy or water supply sector. Other types such as Palmer-based indices (Palmer, 1965), which are based on simple water balance models, or multivariate drought indices (Rajsekhar et al, 2015) might also be suitable in such a context.…”

Can limits of plant available water be inferred from soil moisture distributions?

“…Amidst these emerging needs, the idea of leveraging long‐term soil moisture observations to identify operational brackets of plant available water capacity lends some optimism. This concept has been relied on for assessing soil moisture‐based drought conditions by developing indices (e.g., soil moisture index, soil water deficit, fraction of available water, soil water depletion, fraction of transpirable soil water) that are normalized by soil moisture extremes (AghaKouchak, 2014; Espinoza‐Dávalos et al., 2016; Ford & Quiring, 2019; Ford et al., 2015, 2016; Herold et al., 2016; Krueger et al., 2019; Leeper et al., 2021; Ochsner et al., 2013; Zhao et al., 2020). Extremes defined by 95th and 5th percentiles of θ observations have been used as proxies of θ FC and θ PWP , respectively (Cao et al., 2022; Hunt et al., 2009, 2016; Liu et al., 2017; Martínez‐Fernández et al., 2015).…”