Drought monitoring using a Soil Wetness Deficit Index (SWDI) derived from MODIS satellite data

• A new agricultural drought index has been developed using satellite data as an indicator of soil moisture deficit.
• Spatial and temporal variation of the index has been investigated and compared to regional reports and other vegetation indices.
• Results indicated that SWDI index has the capability of mapping spatial distribution of areas affected by drought, as well as the drought intensity.
• Results also showed that the presence of vegetation plays a modifying role facing with both wet and dry conditions.

Soil moisture is considered a key index of agricultural drought monitoring systems due to its importance for plant growth and biological interactions. In this research, a Soil Wetness Deficit Index (SWDI) was developed based on a Soil Wetness Index to evaluate soil moisture deviation as an indicator of agricultural drought. The Soil Wetness Index is derived using a triangle space concept between the land surface temperature (LST) and vegetation index (NDVI). To acquire the triangle space concept, 8-day-products of land surface reflectance and LST derived from MODIS satellite data over Isfahan were used. The data was collected in the period of 2000–01 (dry year) and 2004–05 (wet year) on an 8-day time step. The results indicated that the SWDI index has the capability of mapping the spatial distribution of areas affected by drought, as well as the drought intensity. The estimated cumulative number of dry days (with −4 < SWDI < 0) in the period of 2000–01 was 184 days. The results also confirmed the existence of wet days in the period 2004–05. Moreover, shifts in drought condition at the end of the wet and dry periods were detected in the area. Results also showed that the presence of vegetation plays an important role in balancing soil moisture variation.