A drought index designed for field-scale water management

A drought index with respect to the spatio-temporal scale was developed in response to the demand from the agricultural sector in South Korea. The new drought index was calculated based on the soil water balance between the supply and demand of water. The water supply was estimated using the cumulative effective precipitation weighted by the precipitation from two months prior. The water demand was derived from the actual evapotranspiration, which was calculated by applying a crop coefficient to the reference evapotranspiration. The amount of surface runoff on a given soil type was used to calculate the residual soil moisture. The presence of drought was determined based on the probability distribution in the given area. In order to assess the reliability of this index, termed the Agricultural Drought Index (ADI), the amount of residual moisture, which represents the severity of a drought, was compared to the measurements of soil moisture at three experimental sites between July 2012 and December 2013. The results showed that the ADI had greater correlation with measured soil moisture than did the Standardized Precipitation Index (SPI), suggesting that the ADI is a useful indicator of drought. While both the SPI and ADI showed similar trends in the temporal variation of drought conditions at all of the sites, the ADI better detected ‘severe drought’ than did the SPI. The daily relief of severe drought due to precipitation was also better represented by the ADI. Using the high-resolution climatic and spatial data of a small watershed, we produced 270m resolution maps of ADI from week 36 through week 41 of 2013, demonstrating the feasibility of the ADI as an operational drought monitor appropriate for the agricultural environment of South Korea.