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University of KwaZulu-Natal (2019)

Soil water dynamics and response of cowpea to water availability under moisture irrigation

Kanda, Edwin Kimutai.

Titre : Soil water dynamics and response of cowpea to water availability under moisture irrigation.

Auteur : Kanda, Edwin Kimutai.

Université de soutenance : University of KwaZulu-Natal

Grade : Doctor of Philosophy (PhD) 2019

Résumé partiel
Increasing population, urbanization and industrialization has put pressure on the irrigation sub-sector to produce more yield using less water i.e. improving crop water productivity (WP). This can be achieved through the adoption of efficient irrigation systems such as micro-irrigation. Moistube irrigation (MTI) is a relatively new technology like subsurface drip irrigation (SDI) but with a semi-permeable membrane whose nanopores emit water in response to applied pressure and soil water potential. Being a new technology, there is little information regarding its hydraulic characteristics and soil water distribution which are necessary for its design, operation and management. Furthermore, the response of crops under a variety of soils and environmental conditions under MTI has not been covered extensively. Therefore, this study aimed at determining the hydraulic and clogging characteristics of MTI. The effect of soil texture on the soil water dynamics of MTI was also determined. Finally, the response of cowpea, an important but neglected African indigenous legume, to varying water regimes under MTI was also determined. This study was based on the hypothesis that cowpea responds favourably to water regimes under MTI. The study was accomplished through laboratory, field experiments and agro-hydrological models. AquaCrop and HYDRUS 2D/3D were chosen for this study due to their reliability in predicting crop yield responses to water availability and soil water dynamics respectively. The laboratory experiments were conducted in soil bins to determine the soil water dynamics of MTI under sandy clay and loamy sand soils which were used to calibrate the HYDRUS 2D/3D model. The hydraulic characteristics were determined at a pressure of between 10 kPa and 100 kPa while the effect of suspended and dissolved solids was determined under a pressure of 20 kPa and 30 kPa. The field experiments consisted of glasshouse and tunnels to examine the response of cowpea to full and deficit irrigation of MTI with SDI as the control. The results were used to parameterise and validate the AquaCrop model. Finally, HYDRUS 2D/3D and AquaCrop were coupled to draw into the strengths of the individual models and used to simulate the water use of cowpea under MTI in two agro-ecological zones in South Africa. The results showed that the discharge – pressure relationship of Moistube followed linear and power functions. It was also established that suspended solids had severe clogging effect than dissolved solids. In the soil bin experiment, simulated water contents closely matched (R2 ≥ 0.70 and RMSE ≤ 0.045 cm3 cm-3) the observed values in all the points considered for the two soil textures. The model slightly under-estimated or over-estimated the soil water content with percent bias less than 15.6%. There was no significant difference (p > 0.05) between the soil water distribution in lateral and downward direction for both sandy clay loam soil and loamy sand. However, the soil water content upward of the Moistube placement depth was significantly lower (p < 0.05) than both the lateral and downward soil water contents in loamy sand.


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