Assessment of groundwater dynamics for agricultural use in six inland-valley agro-ecosystems of SW Nigeria. (96)
Keywords:Wetland, inland valley, fadama, groundwater, piezometers, food security, agriculture.
AbstractInland valley (IV) agro-ecosystems have the potential for sustaining the intensification and diversification of agricultural production relative to uplands. This study investigated the groundwater table dynamics of six inland valleys in SW Nigeria in relation to cropping. The valleys are of three types: amphitheatre-like valley-heads (Am), valley-side (VS), and low depression (LD) inland valleys, with the hydromorphic zone ranging between 1690 – 28547 square metres. Piezometric data were used to present the groundwater table dynamics. Results of analysis of time series of groundwater flow showed a decreasing wetness condition in the order of Am > VS > LD. The monthly average depth to the water table during the wet season ranged between 2 to 27.2cm, 10 to 3cm and -2.5 (above the ground surface) to 57cm in Am, VS and LD inland valley types, respectively. The ranges were 7.5 to 21.5cm, 18.9 to 32.9cm and 25 to 125.1cm in Am, VS and LD valleys respectively, during the dry season. Analysis of the pattern of groundwater level duration plots indicates that the Am valleys had piezometric heads generally above 30cm, with a mean monthly range of 2 to 29cm. The VS valleys had periods of low (less than 30cm) and deep (more than 30cm) groundwater level for 90% and 10% of the days in the hydrologic year, respectively. Compared to the Am and VS valleys, the LD valleys had shorter period of low groundwater level (about 65%) and longer period of deep groundwater level (about 35%). Groundwater level duration curves revealed variable flow regime along inland valleys typologies. Though ANOVA results indicated no significant (α = 0.05) difference between depth to the water table in the valleys during the wet season, there was a significant (α = 0.05) difference during the dry season. The amphitheatre-like and valley side inland valleys were always wet enough to support dry season farming with little or no supplementary irrigation, while application of such irrigation would be required in the low depression valleys to enable dry season cropping.