Assessing the length of growing season and drought incidence in Bihar (original) (raw)

Drought is a serious meteorological hazard in Bihar affecting the successful crop production. The recent two consecutive disastrous droughts during 2009 and 2010 in the state are a stark reminder for the scientists and policy makers alike to sustain agricultural production in the state. This assumes greater significance as more than 80 percent of the population in the state depends on agriculture and its allied sectors for livelihood. In order to sustain and enhance agricultural production against the weak monsoon, frequent dry spell condition and other extreme events, suitable mitigation and adaptation strategies need to be worked out and be taken up in the planning against drought. The variability of rainfall in both space and time affects the effective growing period. At a given location, soil type, available water holding capacity and evaporative demand determine the length of the growing season. The water availability to crops under rainfed condition was determined following agro-meteorological procedures in several regions (Ramana Rao et al., 1979; Gupta et al., 2010). Flexible time scale has been attributed as the fundamental strength of standardized precipitation index (SPI) to study drought over other indices (Mishra and Singh, 2010). Analyzing long term rainfall, Kokilavani (2012) determined the length of growing period for crop planning in Coimbatore district. Looking to every facet of the drought situation, moisture availability periods are required to be characterized for proper assessment of rainfall resource towards developing effective management strategy for its optimum utilization for crop production under rainfed condition. In this paper, an attempt has been made to assess the length of growing period and drought incidence and vulnerability in Bihar. The study was conducted in various districts under different agroclimatic zones of Bihar. The state is located between 24°17' and 27°31' N latitudes and between 83°19' and 88°17' E longitudes covering an area of 9.38 million hectares. It is broadly divided into three agro-climatic zones viz. Zone I (North West Alluvial plains), Zone II (North East Alluvial Plains) and Zone III (South Bihar Alluvial Plains). Zone III is further subdivided into Zone IIIA and Zone IIIB on the basis of rainfall variability and topography. Annual rainfall characteristics and length of growing periods of various locations in different agroclimatic zones were worked out. Reddy (1993) used a simple tool to compute the length of growing period for a semi arid area of the tropics based on mean weekly rainfall and mean weekly PET. In the present study, the ratio of mean annual rainfall to mean annual PET was used to determine the types of climate that prevail at a particular location. When this ratio at a particular location ranged between 0 to 0.25, the climate of that place was designated as arid climate, 0.25 to 0.50 as semi arid climate, 0.50 to 0.75 as sub-humid climate and 0.75 to 1.0 as humid climate. When the ratio exceeds the value of 1.0, the climate of that particular location is regarded as per humid climate. Length of growing season (LGP) which is defined as the period during which the moisture at the root zone of crop plants is adequate to meet the water need was calculated as, LGP= [Duration of rainy season in days + (Post monsoon and winter rainfall, mm+ AWHC, mm) /3.0] Average evaporative demand of the atmosphere from agricultural field was assumed as 3.0 mm day-1. As per soil characteristics, the available water holding capacity (AWHC) per metre depth for Pusa, Patna, Sabour and Bikramganj soils was assumed as 150 mm. The AWHC of Motihari and Madhepura soils were taken as 200 mm and 100 mm for soils at Nalanda. Standardized Precipitation Index (SPI) values for various districts have been worked out using daily rainfall data of individual district for monsoon season over a period of ten years (2000-2010). SPI was calculated as the total difference of rainfall for a given period from its long term mean and then normalized by the standard deviation of precipitation for the same period computed using data over the analysis period. A drought starts when SPI values reach-1.0 and ends when SPI becomes positive again.