Estimation of rainfall distribution and its relation to rice production in Laos



Figure 3. Verification of rainfall estimation respect to the points which were not applied for semi-variogram modeling.
( Data set : Recorded by data logger in 2001 & 2002).


3.2 Estimation of rice production by rainfall factor
Two kinds of multi regressions to estimate harvested area and yield of lowland rice were obtained (Table 4). Contribution of dummy variables was significant in both regressions, and monthly rainfall variables from May to October were also selected to estimate harvested area. Monthly rainfall in July and time trend variables were selected to estimate yield. The squared multiple correlation coefficient adjusted for degrees of freedom (Adjusted R-square) was 0.9470 for the harvested area and 0.5632 for yield. In the regression to estimate the harvested area, rainfall variables for May, June, and July showed a positive coefficient but rainfall in August and September showed a negative coefficient. This implies that rainfall in the late rainy season risks cause damage to rice production by decreasing the harvested area.

The regression for estimation of harvested area, which gave a better correlation, was applied to map calculation by GIS using the rainfall distribution maps produced in 3.1, to illustrate harvestable area per person (Figure 4). The expected harvestable area in the northern region was consistently less than 0.1 ha per person


Figure 2. Estimated rainfall maps by Kriging interpolation with semi-variogram.


Table 3. Mean rainfall in province estimated by Kriging with semi-variogram (mm).


through 12 years, but the central and southern regions varied according to the rainfall conditions each year. In Champasak Province, for instance, the expected harvestable area was comparatively large until 1997, but decreased in 1998, slightly increased in 1999 and 2000, then decreased in 2001 and 2002 again. In Saravane Province, a large harvestable area was expected through 12 years, but the western portion was smaller than the eastern portion in 1996 and 2000. Thus, spatial association between rainfall and rice production was shown by the grid image.

4. Conclusion
This paper investigated the relation between rainfall and rice production in Laos using spatial analysis. Domestic rice production in Laos increased from 660,000 ton in 1976 to 2,530,000 ton in 2004 (Dep. of Planning, 2004). Although irrigation developed after the 1990s, rice production by irrigation accounts for only 16% of total production. On the other hand, rice production by rainfed agriculture is steady at 77% of overall production. In addition, upland rice is a primary crop in mountainous regions known to be high-poverty areas. Accordingly, rainfed paddies and upland fields are still crucial to both self-sufficiency and livelihood in Laos. In regions where socio-economic infrastructure and distribution networks have not been sufficiently developed, it is important not only to increase total countrywide rice production but also to stabilize regional production in each region. Rainfall cannot be controlled; however, it should be possible to devise countermeasures to fluctuations by employing farming methods suited to regional rainfall conditions. A better understanding of annual / regional fluctuations and the spatial correlation between rainfall and rice production is likely to provide useful information.


Figure 4. Potential harvestable area expected by rainfall condition in each year (per person).


References

  • Agricultural Censes Office (2000) Lao Agricultural Census, 1998/99 Highlights. Vientiane
  • Dept. of Meteorology and Hydrology,Dairy Recorded Rainfall of Lao P.D.R. 1991-1996; 1999; 2002; 2003..
  • Dept. of Planning in Ministry of Agriculture and Forestry,2004,Agricultural Statistics Year Book 2004.
  • Jayawardene,H.K.W.I., Sonnadara, D.U., Jayewardene, D.R., 2005, Spatial interpolation of weekly rainfall depth in the dry zone of Sri Lanka. Climate Research 29, pp.223-231.
  • Robertson, G.P., 1987, Geostatistics in ecology: Interpolating with know varianve. Ecology 68(3), pp.744-748.
  • Yamamoto, Y., Furuya J., Suzuki K., Ochi S., xxxx, Characteristics of Rainfall and Rice Production in Laos Described by Spatial Analysis, Jl. Of the Japanese Agricultural Systems Society [In Japanese] (contributing).
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