Comparison of Drought Probability Assessments Based on Atmospheric Water Deficit and Soil Water Deficit

Abstract

Methods that estimate the probability of agricultural drought using atmospheric data can be widely applied but have not been compared with actual drought occurrence indicated by soil moisture measurements. Our objectives were to develop a drought probability assessment method using long‐term measurements of soil water deficits (SWDs) and to compare the resulting probabilities with those of an existing method based on atmospheric water deficits (AWDs). Fifteen years of daily precipitation, air temperature, and soil moisture measurements for eight locations across Oklahoma were used to calculate the probability (P) of water deficits sufficient to cause plant water stress for each day of the growing season. For the SWD method, the drought threshold was set at 50% depletion of the soil’s total available water capacity. For the AWD method, the threshold was a 7‐d cumulative AWD of 50 mm. Values of AWD were similar to calculated SWD for the 0‐ to 40‐cm soil layer in the spring; however, AWD values seldom reached the drought threshold. Thus, drought P values calculated by the AWD method were unreasonably low and consistently lower than P estimated by the SWD method. The AWD method showed greater agreement with the SWD method when 37 mm was used as the AWD threshold or when the original 50‐mm threshold was applied for a 15‐d cumulative AWD. The new SWD method gave plausible and consistent results when applied to both the 0‐ to 40‐ and 0‐ to 80‐cm soil layers and should be utilized when long‐term soil moisture data are available.

Evaluation of the SMOS-Derived Soil Water Deficit Index as Agricultural Drought Index in Northeast of Brazil

“…In line with the results from some previous studies [26,28,36,67,75,76], it was found that the atmospheric conditions over NEB influences the upper soil water balance dynamic. This characteristic is reflected by a moderate coupling strength between the AWD and SWDIS from June 2010 to December 2013 in the entire NEB (Figures 2 and 3).…”

“…This water is stored not only in the surface layer but also in the root-zone layer [26]. Although some studies have demonstrated the presence of a strong correlation between the content of moisture in surface and root-zone [67,82], the coupling strength among soil layers decreases as depth increases, and moreover depend on the prevailing hydrometeorological conditions [36,83,84].…”

“…In order to verify the impact of droughts on the upper soil moisture of the NEB, daily values of the Atmospheric Water Deficit (AWD) were used. The calculation of the AWD require as inputs, observations of daily rainfall and daily potential evapotranspiration (ETo) [26]. For this study, four grids with a spatial resolution of 0.25 • , of daily rainfall, ETo, maximum and, minimum temperature for the entire Brazil throughout the period of 1980 to 2013 was used.…”

“…AWD is a suitable tool to identify the drought dynamics related to soil water storage [26,67]. For its calculation, the cumulative ETo for the preceding 6-day and the day under consideration are summed (7-day total), and the 7-day cumulative precipitation is subtracted from this value, resulting in a 7-day cumulative AWD estimate for each day of the long-term record.…”

“…It is worth be mentioned that the upper soil moisture is closely linked to atmospheric water deficit [26,36,67]. For this reason, a linear correlation analysis can be used to explore the temporal relationship between both signals.…”