Spatial Variations in Water Availability, Soil Fertility and Grain Yield in Rainfed Lowland Rice: A Case Study from Savannakhet Province, Lao PDR

Rice is the single most important food crop in Laos. Savannakhet province, the largest area of rainfed lowland rice of any single province in the country was selected for the present case study to quantify the spatial distribution of two major limiting factors, water availability and soil fertility, and rice productivity in rainfed lowlands. Field water availability, fertilizer application and other crop management practices, and grain yield information were collected from over 100 farmers to provide basic information relating to rainfed lowland rice productivity and potential bio-physical constraints in this province over two rice cropping seasons. Poor soil fertility is identified as a major yield constraint with yield responding strongly to fertilizer application rate. The results also show that rainfall distribution pattern, soil type and position of rice fields on a sloping land, affect paddy water availability, and this in turn influences sowing time and is also expected to have effect on grain yield. To improve the productivity of rainfed lowland rice, combination of appropriate crop phenology, increased fertilizer use that is matched with water availability, and an understanding of soil water condition for the rice growing environment, is required. In a long term increasing soil fertility is required, and a significant improvement in rice productivity cannot be achieved by improved water availability alone, in a situation where the majority of paddy soils have low levels of fertility

Rainfall variability and its effects on growing period and grain yield for rainfed lowland rice under transplanting system in Northeast Thailand

Rainfall variability in Northeast Thailand during 2000-2015 was examined with objectives to determine any changes in rainfall pattern with time, and to determine its effects on duration of rice growing period and grain yield using a simulation model. Variation in mean annual rainfall over 16 years in 93 locations in the region ranged from over 1,600 to less than 1,200 mm, and the locations were grouped into 4 based on the annual rainfall. The change in annual rainfall, and early, mid and late season rainfall was analysed for the rainfall groups. There was a significant reduction in the amount of early season rainfall during the 16-year period in all groups. However, there was no significant change for annual, and mid and late season rainfall. Simulation study showed that the start of rice growing period (SGP) was delayed with reduced early rainfall during the 16 year period and the end of rice growing period (EGP) was also delayed while there was no significant change for the length of rice growing period (LGP). Simulation results showed that grain yield of KDML105, leading variety in Thailand, tended to increase during the 16 year period, as delayed planting time was optimum for achieving maximum yield in all rainfall groups. With general delay in rainfall season, occurrence of late season drought was predicted to be reduced and this helped to increase simulated grain yield. However, adaptation to changing rainfall pattern needs to be planned in advance to maximize its effect

Quantification of an overlooked water resource in the tropical rainfed lowlands using RapidEye satellite data: a case of farm ponds and the potential gross value for smallholder production in southern Laos

In southern Laos, water stored in farm ponds is largely underutilised as it is perceived to be unfit for human consumption; subsequently, groundwater is the preferred source for domestic and agricultural consumption. For the first time, this paper presents the results of a study designed to quantify the total pond water volume within the landscape via remote-sensing methods in two districts in Champasak province that could be used to improve rural household cash income through the expansion of market-oriented dry season crop production. Water bodies were delineated via simple classification of RapidEye data using the Normalised Difference Water Index and a sub-classification was performed to distinguish between ponds and the streamflow network. Final estimates of total pond volume in Sukhuma and Phonthong districts were ∼2.30 × 10 m and 3.55 × 10 m, respectively; and the average pond volume across both districts was ∼1987 m. Sensitivity analysis of the potential gross value of farm ponds for irrigation of dry season, vegetable production typical of market-oriented smallholder activities in the area indicated that substantial gross economic gains could be made from better use and management of these resources