Monitoring of seasonal dryness/wetness conditions using shortwave angle slope index for early season agricultural drought assessment

Abstract

In the present study, shortwave angle slope index (SASI) was derived from near-infrared (NIR), shortwave infrared 1 (SWIR1) and shortwave infrared 2 (SWIR2) bands of Moderate Resolution Imaging Spectroradiometer (MODIS) 500-m 8-day composite images in three meteorological sub-divisions of India. The SASI, sensitive to surface wetness, was found to be strongly correlated with rainfall and its seasonal profiles were able to distinguish the early season drought incidents. Comparison between normalized difference vegetation index (NDVI) and SASI showed that during the beginning of the crop season, SASI was relatively better in discriminating the surface dryness conditions prevailing in different years. The SASI, integrated over the period of June to August, better captured the changing dryness/wetness patterns in different years. A normal SASI profile was developed by averaging the SASI values of two normal years. Integrated SASI (InSASI) was computed for the normal SASI profile. Week-wise area under a SASI curve was computed. Based on deviations from the area under the normal SASI curve, dryness/wetness patterns in terms of normal, moderate dry and severe dry were mapped for June, July and August, separately. These patterns are in good agreement with India Meteorological Department rainfall deviations. The study concludes that the early season/sowing-period agricultural drought assessment based on SASI and InSASI could complement the existing vegetation index-based agricultural drought monitoring mechanism.