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Rainwater harvesting and management (RHM) systems are increasingly being recognized as one of the strategies of upgrading rainfed agriculture, especially by smallholder farmers in semi-arid environments. The need to increase crop yields has led to adoption and up-scaling of RHM systems such as conservation tillage and on-farm ponds for micro-irrigation. The paper focuses on conservation tillage, which refers to soil tillage practices that minimize soil manipulation, increase moisture storage and reduce soil and water loss. It aims at conserving rainwater in situ-where it falls within the root-zone. Increased soil moisture storage reduces runoff, which lead to reduction in catchment water yield, and may reduce river flows during the wet and dry seasons. Hence the need to estimate the amount of rainwater retained on croplands and related hydrological impacts. However, estimation of the amount of runoff-additional soil moisture-retained due to in situ RHM systems such as conservation tillage practices is not straightforward.

The paper presents an empirical approach based on the analysis of crop yield response to water. The result from empirical approach was compared with field data taken from croplands under traditional and conservation tillage systems in Laikipia district, Kenya. Additional soil moisture retained in the root-zone was reflected in increased maize biomass and grain yields. The empirical approach assumed other soil water balance components such as deep percolation, overall reduction in soil evaporation by conservation tillage over the cropping season and long-term change in soil moisture storage are negligible. The approach is simple compared to other methods based on costly and time consuming field instrumentation and measurements. Field measurements showed that conservation tillage could increase soil moisture storage by 18-50%. Empirical approach yielded values within the same range, i.e. 15-40%. Therefore, empirical approach can be used to estimate the amount of rainwater retained on croplands, and hence proportion of runoff reduced from agricultural catchments. (c) 2006 Elsevier Ltd. All rights reserved.