Plant and Soil 263: 165–171, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 165 Water use efﬁciency of a maize/cowpea intercrop on a highly acidic tropical soil as affected by liming and fertilizer application T. Gaiser 1,4 , I. de Barros 2 , F.-M. Lange 1 & J.R. Williams 3 1 University of Hohenheim, Institute of Soil Science and Land Evaluation, 70593 Stuttgart, Germany. 2 CIRAD, TA80/01, Avenue Agropolis, 34398 Montpellier Cedex 5, France. 3 Texas Agricultural Experiment Station, Temple, Texas, USA. 4 Corresponding author ∗ Received 21 February 2003. Accepted in revised form 5 December 2003 Key words: acid soils, cowpea, maize, fertilization, liming, water use efﬁciency, soil water balance models Abstract Due to global warming, there is a need to increase the water use efﬁciency of crops under rainfed agriculture, particularly in semi-arid regions. Therefore, the effect of NPK fertilizer application (with or without liming) on the water use efﬁciency of a maize/cowpea intercropping system was investigated in the semi-arid part of Brazil. The crops were grown on a strongly acidic, sandy soil with three treatments: (i) Complete NPK fertilizer application with lime (Compl), (ii) Complete NPK fertilizer application without lime (Compl-L) and (iii) Control. On the average, dry matter production was 2.6 times higher with the Compl treatment than in the Control and 1.6 times higher than in the Compl-L treatment. The soil water balance was calculated with two different model approaches (HILLFLOW and EPICSEAR). When checked against measured soil water content during the growing period, both models produced accurate results, but only EPICSEAR was sensitive to the effects of liming and fertilizer application on soil water balance and dry matter production at this site. Comparison between the Compl and the Compl-L treatments shows that the increase in transpirational water use efﬁciency (WUE T )(+63 and +80%, respectively) is mainly due to the application of NPK. Although the site is highly acid, liming was of minor importance for increasing the WUE T . However, observations and simulations demonstrate that, through the additional application of lime, the gross water use efﬁciency (WUE C ) in a maize/cowpea intercropping system can be increased by 60% compared to sole application of NPK and by more than 160% compared to the control. Abbreviations: EPIC – erosion productivity impact calculator; EPICSEAR – erosion productivity impact calculator for semi-arid regions; TDR – time domain reﬂectometry; WUE – water use efﬁciency. Introduction In future, water will become increasingly scarce par- ticularly in semi-arid regions. There, global climate change may lead to higher potential evapotanspiration, decreasing precipitation and increasing frequency of high intensity rains. At the same time, water demand is most likely to grow due to higher population density and expanding areas of irrigation (IPCC, 2001; Shik- lomanov, 2001). Hence, there is an urgent need to use water more efﬁciently in these regions. ∗ FAX No.: +49-711-4594071. E-mail: tgaiser/@uni-hohenheim.de To the agronomists, water use efﬁciency (WUE) is often deﬁned as yield per unit area divided by the water consumption to produce the yield. However, there are various ways in expressing yield (grain yield or marketable yield or dry matter yield) and water consumption (total water input or actual evapotran- spiration or crop transpiration). This makes it difﬁ- cult to compare water use coefﬁcients from different sources (Sinclair et al., 1984). When WUE is related to the total water input or to the sum of evaporation and transpiration, there are several management tech- niques for changing WUE by reducing non-productive losses through soil evaporation, runoff and drainage