MODELLING THE EFFECTS OF IRRIGATION DEPTHS AND DRAINAGE SYSTEMS ON RICE PERFORMANCE IN THE GUINEA SAVANNAH AGRO ECOLOGICAL ZONE OF GHANA

Abstract

The study modelled the effects of irrigation depths and drainage systems on the performance of rice in the Guinea Savannah agro-ecological zone of Ghana. The study specifically evaluated the effect of different irrigation application depths and drainage systems on growth and yield of rice, assessed the effects of irrigation application depths and drainage systems on changes in soil electrical conductivity, soil temperature and nitrogen balance in irrigated ecology, estimated Crop Water Stress Index (CWSI) for rice culture in an irrigated ecology and modelled the effects of irrigation application depths and drainage systems on LAI and yield of rice. The irrigation application depths were continuous flooding at 5 cm, 10 cm and 15 cm and alternate wetting and drying (AWD) at – 5 cm, - 10 cm and – 15 cm while the drainage systems were no drainage as control, surface drainage and sub-surface drainage systems. Micro plots with concrete blocks were constructed measuring 1 m × 1 m × 1 m and laid in a Randomized Complete Block Design in a 6 × 3 factorial treatment arrangement replicated three (3) times in 2023 and 2024 growing seasons. Soil electrical conductivity (EC) was used to determine the salinity index of the soil. Monitoring of temperature within the root zone of the crop was done throughout the crop growth cycle depicting each of the six stages in the paddy rice. To compute the baselines, the air temperature, leaf canopy temperature and the vapour pressure deficit (VPD) were used. Results indicated that sub-surface drainage with 5 cm irrigation depth produced the tallest plants (80 – 94 cm), high LAI (2.39 – 3.89), high leaf chlorophyll content (16.24 – 19.93 CCI) and a high yield (6.77 – 9.55 t/ha). No drainage under AWD 15 recorded the shortest plant (65 – 79 cm), lower LAI (0.85 – 2.16), lower Leaf Chlorophyll Content (LCC) (5.37 – 9.50) and lower yield (0.41 – 1.27 t/ha) in both seasons. The salinity level of the soil was high (347.8 µS/cm) in treatments with no drainage and low (186 µS/cm) in sub-surface drainage. Soil temperature results indicated that surface drainage under AWD 15 recorded the highest (37 – 43 oC) while the lowest (20 – 26 0C) was recorded in treatments with no drainage under continuous flooding. AWD with sub-surface drainage resulted in higher nitrogen content (0.064 – 0.095 %) while the lowest was recorded in continuous flooding with no drainage (0.038 %) and continuous flooding with surface drainage (0.031 %). Results indicated that CWSI was lower (0.075) in CF 5-10 in 2023 and 0.143 in 2024 due to temperature differences while the highest CWSI (0.831 and 0.857) were recorded in AWD – 15. In 2023, CWSI gave a regression model of y = - 0.1191x + 0.7587 and a coefficient of determination (R2) value of 0.987 while in 2024, a regression model of y = - 0.0969x + 0.8673 where (x) represents the irrigation application depths. Observed simulation showed more accurate results for CF 5 with no drainage (d-stat = 0.92 and R2 = 0.93). The regression analysis of the yield produced a regression equation y = - 0.242x + 6182.2 with a coefficient of determination (R2) value of 0.654. During the evaluation, R2 for no-drainage condition ranged from 0.81 – 0.97, RMSE ranged from 0.11 – 1.72 and D-index ranged from 0.31 – 0.98. The continuous flooding at irrigation water depth of 5 cm (CF 5) gave the highest Willmot’s d-index of agreement of 0.98 while the lowest d-index was recorded on treatment AWD -15. In conclusion, sub-surface drainage system under 5 cm irrigation depth proved to be more efficient in terms improving the growth and yield parameters of rice while at the same time conserving water. The values derived for CWSI in this study can be used to understand the stress dynamics of rice in various stages of growth under different irrigation depths and drainage systems. The performance of rice can be increased greatly by adopting irrigation application depth of 5 cm with drainage management being put in place. CWSI estimation offers a practical tool for drought mitigation and irrigation scheduling in both irrigated and rainfed rice systems.