Spatio-Temporal Variability of Water Quality and its Response on Growth of Irrigated Rice in Rusurirwamujyinga Sub-catchment, Rwanda.
The water quality used for irrigation is essential for the yield and quantity of crops, maintenance of soil productivity, and protection of the environment. Irrigation water quality has a profound effect on soil and plant health. A major concern with water used for irrigation is decreased crop yields and land degradation as a result of poor water and soil management which lead to irrigation induced problems, such as salinity and water-logging capacity. The Rusurirwamujyinga rice irrigation scheme of Rusurirwamujyinga sub-catchment in Rwanda was subjected to a study of its water quality and its response on growth of rice. The aim of this study was to assess spatial and temporal water quality variability and its response on growth of irrigated rice in Rusurirwamujyinga sub-catchment. Specifically, this study analysed physico-chemical parameter in the water used for rice irrigation by considering stream positions (upstream, middle and downstream) and seasonal variation (dry season, moderate and rainy season) in order to find out the variation of irrigation water quality within time and space. The study also analysed the status of soil fertility and rice plant nutrients in order to determine the trend of relationship between water quality, soil and plant nutrients. Data collection methods used included the primary data obtained from water, soil and plant through analytical laboratory and field measurements and secondary data was obtained from the meteorological station, topo sheet maps and review of available literature. The statistical tools used to analyse data were mainly descriptive statistics and correlation analysis in order to determine central tendency, dispersion, reliability and variability of the data. The index of water quality for irrigation and general ANOVA and regression ANOVA LSD at (P < 0.05) was used to estimate the significant difference of water quality between seasons (dry season, short rainy and short dry season and rainy season), between stream positions (upstream, middle and downstream) and to estimate interrelationship between water quality, soil and plant nutrients. The experimental results showed significant variation of irrigation water quality both in season and in stream positions. No significant interrelationship between water quality, soil and plant nutrients was observed in all variables tested in water, soil and rice plants. All parameters measured were recommendable for use without any impact to crops when compared with the FAO guidelines of evaluation water quality for irrigation. The correlation matrix between water quality parameters showed moderate positive and negative correlation (r = 0.4 – 0.6), weak positive and negative correlation (r = 0.2 – 0.4) and no correlation (r = 0 – 0.2) between parameters. The study makes the following recommendations: Rusurirwamujyinga sub-catchment needs to be sustained by reducing the negative impact which causes water pollution. The recommendations were given to the management of Rusurirwamujyiga dam as well as the whole irrigation infrastucture, which is used for supplying and regulation of water for irrigation. The use of Best Management Practices (BMPs) to reduce agriculture‟s impact on water quality while enhancing agricultural production in the irrigation scheme will be the best way of soil and water management by ensuring quality and equitable sharing of the water resource in the study area.