Estimation of rainfall distribution and its relation to rice production in LaosYukiyo YAMAMOTO Jun FURUYA Kenji SUZUKI Shiro OCHI Rainfall in Laos is brought by the southwest monsoon from May to October; annual rainfall ranges from 1300–1800 mm. Rice is the primary crop in Laos and approximately 80% of rice production depends on rainfed agriculture during the rainy season; however, drought and flood owing to erratic rainfall can cause unreliable production. To clarify the regional rainfall characteristics, monthly rainfall distribution for 1991 to 2002 was estimated by Kriging interpolation using a semi-variogram representing spatial correlation among 72 rainfall stations. The resolution of estimation was 1 km. Estimation error to the observed dataset was within 30 mm, equivalent to less than 5% of each monthly rainfall figure. In addition, the correlation coefficient of estimation for 100 samples to which Kriging interpolation was not applied was 0.7902. The harvested area and yield of lowland rice for 1991 to 2002 were estimated by multi regression analysis with rainfall variables. The adjusted R-square values were 0.9470 and 0.5632 respectively. In the regression to estimate harvested area, rainfall variables in May, June, and July showed positive coefficients, and August and September showed negative coefficients, implying that rainfall in the late rainy season reduces harvest area. The equation was finally applied to map calculations in GIS to illustrate the expected harvestable area per person by rainfall pattern in each province. 1. Introduction Laos occupies the largest area of 6 countries located in the Mekong River basin. More than 50% of GDP depends on agriculture and forestry. By the census data in 1998/1999, 668,000 households, corresponding to 83.7% in all of households in Laos, are agricultural holdings; however, most are primarily subsistence farms that produce for their own consumption (Agricultural Census Office 2000). Rice is the staple diet and primary crop in Laos. Various farming types such as rainfed paddies, lowland irrigation and slash-and-burn uplands have been extensively exploited for rice production. The areas of rainfed, irrigated, and upland rice fields in 2004 were 575,520 ha, 76,840 ha, and 117,960 ha respectively. More than 80% of rice is produced by rainfed agriculture (Dept. of Planning, 2004). Rainfall in Laos is brought by the southwest monsoon from May to October and annual rainfall ranges from 1300–1800 mm; however, fluctuation by year and region is observed. Droughts and floods are significant factors in unreliable unstable production. The aim of this study was to reveal the relation between rainfall and rice production, both spatially and quantitatively (Yamamoto et.al. xxxx). 2. Data and methodology 2.1 Estimation of rainfall distribution  Rainfall distribution can be estimated by spatial interpolation of rainfall data at points of observation. Meteorological monitoring in Laos has been conducted by the Department of Meteorology and Hydrology at the Ministry of Agriculture and Forestry; they publish an annual data book of 104 rainfall gauges (Dept. of Meteorology and Hydrology). We selected 72 points for the study which had a time series dataset after 1991 and latitude/longitude information (Figure 1). Monthly rainfall data in 72 stations in the rainy season from May to October for 1991-2002, i.e., a 72-month dataset consisting of 6 months in 12 years, was converted into point data format with an attribute table in GIS.
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