Rain lag (dry spell) and probability-based rainfall on dry land typology in Lombok
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Abstract
Dry spell and probability of precipitation are very essential for both rainfed and the dry land agriculture. The parameters are important indicators of drought status and have relevance to crop risk in dry land areas. The research objective was to determine duration of dry spells and probability-based rainfall in the area of climate types D and E at southern Lombok. Descriptive research was conducted in 2014 by collecting daily rainfall data from 13 rainfall stations. The stations were assigned as purposive sampling to represent the southern part of Lombok area. Conditional upon the nature of the incident rainy day or day without rain was analyzed using Markov chain model of first order, using daily rainfall data of 16 years. Probability of precipitation was determined using a formula of transformation in order to meet the normal curve distribution properties. Drought-level assessment was carried out based on the value of the standard precipitation index, SPI using growing season rainfall data of span 30 years. The results show, that the dry spells and probability-based rainfall varied spatially and temporally as well as they were influenced by variation in the standard precipitation index; dry spell was longer getting to the east, while rainfall decreased. Dry spell was parabolic in nature; It is always longer in the early growing season (November) and at the end of the season (April), and It was relatively short in between. Dry spell on SPI-negative was longer than that on SPI-positive. The characteristic of the growing season tends to be getting drier, with a level of mild drought over all.
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