Crop yield and water use in smallholder irrigated Agriculture: a case study of Nyanyadzi (Zimbabwe) and Matanya (Kenya) irrigation schemes
The performance of most smallholder irrigation schemes has been disappointingly low. This study aims at establishing the causes of low crop yield in Nyanyadzi and Matanya irrigation schemes and to identify possible interventions. Nyanyadzi irrigation scheme was developed in 1938. Maize crop yields are between 0.86-2.5 t/ha compared to a potential of 4-5 tons/ha. Matanya irrigation scheme was developed in 1940 and its crop yields for maize are between 0.5-1 t/ha compared with a potential of 5-6 t/ha. Causes of low crop yields at Nyanyadzi irrigation scheme were established usmg literature review, questionnaire study and field observations. The questionnaire survey was also used to assess farmer perceptions on the on -farm water supply situation and their management practices (among other issues). The main factors limiting crop production were identified in order of their importance as water shortage, low input use (fertilizer and manure), poor irrigation scheduling, low pesticides use, and inadequate technical advice to farmers. The level of input was found to be related to the water availability and to the position in the scheme. As a follow-up to the findings at Nyanyadzi irrigation scheme, maize yield response to water scenario studies were carried out using the DSSAT model under Matanya conditions (also a small-scale irrigation scheme with similar water problems as Nyanyadzi). In this study, 5 levels of irrigation ( 0, 33, 66, 100 and 133 % irrigation requirement) were used and 3 nitrogen levels (0, 50 and 100 kgN/ha ) were used to form a 5*3 factorial experiment with 15 treatment combinations. The model was calibrated using data collected by Ragwa in 1995. Maize crop water requirements and irrigation requirements were determined at. 0, 33, 66, 100 and 133% of the irrigation requirement applied depending on the treatment. Nitrogen was split applied during the vegetative stage, as this is the period when maize growth is most sensitive to nitrogen stress. The results indicated that: 1. Higher yields are attainable with proper water and nutrient management 2. At low levels of nitrogen the yield response to water is very low, that is 1087 kg/ha for rainfed conditions compared to 1387 kg/ha when crop water requirements are fully met. 3. At high levels of water and nitrogen significant yield increases can be attained, from 1087 kg/ha for rainfed conditions and no fertilizer added to 8117 kg for 273 mm of irrigation water and 100 kg of nitrogen. 4. There are benefits in applying less than 100% irrigation requirements if the soil fertility status is improved as evidenced by the 33% and 66% irrigation requirements when 2382 and 5567 kg were obtained at 91 and 181mm of water respectively under 50 kg of nitrogen From this study, it can be observed that investment in water improvement without a corresponding investment in improving soil fertility does no significantly increase yield. There is thus a high yield potential that has not been tapped in most small -scale irrigation schemes.