Criteria based decisions for determining agroclimatic onset of the crop growing season

Abstract

Success of rainfed crop production is highly dependent on timely sowing/planting decisions. Variability in sowing/planting dates between years affect crop planning and thus could decrease farm profitability. Therefore, information on the optimum crop sowing window for the season will facilitate crop planning by farmers and other stakeholders. To determine optimum sowing window over 19 Agro-Ecological Regions (AER) of India, three methods viz., Soil Water Balance (SWB), Depth and modified Morris & Zandesta methods were evaluated to identify the most suitable method for working out the onset dates of the crop growing season. The onset dates of growing season determined by three methods were found to be similar over northeastern (hot sub-humid and warm per-humid eco-region), west coast (hot humid to per-humid), cold and hot arid eco-regions in India. However, differences in onset dates of the crop growing season were observed over hot semi-arid to sub-humid eco-region. SWB method was found to be the most suitable based on validating with observed crop sowing dates, false start and number of undefined onset years criteria. However, as SWB method is data-intensive, onset was determined using an alternative Modified Threshold Combination (MTC) method comprising 40 combinations of threshold values viz. rainfall amount, wet spell duration, dry spell duration and dry spell search period. The onset dates determined by MTC and SWB methods were validated and subsequently, appropriate threshold values were identified to work out onset dates in 19 AERs of India. This study revealed that the onset is not influenced by threshold combinations in per-humid, humid and sub-humid regions. However, in semi-arid and arid regions, critical evaluation of criteria for determining onset is vital to avoid false starts and undefined onset years. Information on agroclimatic onset dates for each AER is useful for improving practical utility/decision making, especially in the semi-arid and arid regions of India.