A watershed approach to upgrade rainfed agriculture in water scarce regions through Water System Innovations: an integrated research initiative on water for food and rural livelihoods in balance with ecosystem functions

Abstract. This paper describes an extreme flood event that occurred in the South Pare Mountains in northern Tanzania. A high spatial and temporal resolution data set has been gathered in a previously ungauged catchment. This data was analysed using a multi-method approach, to gather information about the processes that generated the flood event. On 1 March 2006, extreme rainfall occurred in the Makanya catchment, (300 km 2 ), where up to 100 mm were recorded in Bangalala village in only 3 h. The flood was devastating, inundating large parts of the flood plain. The spatial variability of the rainfall during the event was very large, even in areas with the same altitude. The Vudee sub-catchment (25.8 km 2 ) was in the centre of the rainfall event, receiving about 75 mm in 3 h divided over the two upstream tributaries: the UpperVudee and Ndolwa. The peak flow at the weir site has been determined using the slope-area method and gradually varied flow calculations, indicating a peak discharge of 32 m 3 s −1 . Rise and fall of the flood was very sharp, with the peak flow occurring just one hour after the peak of the rainfall. The flow receded to 1% of the maximum flow within 24 h. Hydrograph separation using hydrochemical parameters indicates that at the floodpeak 50% of the flow was generated by direct surface runoff (also indicated by the large amount of sediments in the samples), whereas the recession originated from displaced groundwater (>90%). The subsequent base flow in the river remained at 75 l s −1 for the rest of the season, which is substantially higher than the normal base flow observed during the previous rainy seasons (15 l s −1 ) indicating significant groundwater recharge during this extreme event.

Section: Study Areamentioning

confidence: 99%

Spatial rainfall variability and runoff response during an extreme event in a semi-arid catchment in the South Pare Mountains, Tanzania

Mul

Savenije

Uhlenbrook

2009

“…In semi-arid populated environments in sub-Saharan Africa (SSA), this is probably even more essential than in other regions since both floods and droughts can result in severe crop loss (e.g. Mul, 2009;Rockström et al, 2004). A good understanding of the hydrology of a semi-arid catchment can help to increase the productivity in smallholder farms and to sustain long-term food security.…”

Section: Introductionmentioning

confidence: 99%

On the value of combined event runoff and tracer analysis to improve understanding of catchment functioning in a data-scarce semi-arid area

Hrachowitz

Bohte

Mul

et al. 2011

Abstract. Hydrological processes in small catchments are not quite understood yet, which is true in particular for catchments in data scarce, semi-arid regions. This is in contrast with the need for a better understanding of water fluxes and the interactions between surface-and groundwater in order to facilitate sustainable water resources management in such environments, where both floods and droughts can result in severe crop loss. In this study, event runoff coefficient analysis and limited tracer data of four small, nested sub-catchments (0.4-25.3 km 2 ) in a data scarce, semi-arid region of Tanzania helped to characterize the distinct response of the study catchments and to gain insights into the dominant runoff processes. The estimated event runoff coefficients were very low and did not exceed 0.09. They were found to be significantly related to the 5-day antecedent precipitation totals as well as to base flow, indicating a close relation to changes in soil moisture and thus potential switches in runoff generation processes. The time scales of the "direct flow" reservoirs, used to compute the event runoff coefficients, were up to one order of magnitude reduced for extreme events, compared to "average" events, suggesting the activation of at least a third flow component, besides base-and direct flow, assumed to be infiltration overland flow. Analysis of multiple tracers highlighted the importance of pre-event water to total runoff, even during intense and Correspondence to: M. Hrachowitz (m.hrachowitz@tudelft.nl) high yield precipitation events. It further illustrated the distinct nature of the catchments, in particular with respect to the available water storage, which was suggested by different degrees of tracer damping in the individual streams. The use of multiple tracers subsequently allowed estimating uncertainties in hydrograph separations arising from the use of different tracers. The results highlight the presence of considerable uncertainties, emphasizing the need for multiple tracers in order to avoid misleading results. This study shows the value of hydrological data collection over one whole wet season using multi-tracers to improve the understanding of hydrological functioning and thus for water resources management in data scarce, semi-arid environments.

“…Large-scale irrigation is hardly feasible for a wide range of reasons (Van der Zaag, 2010). The introduction of smallholder system innovations, such as rainwater harvesting, supplementary irrigation from shallow groundwater, small-scale water diversions and crop diversification (Bossio et al, 2011;Mul et al, 2011;Ngigi et al, 2005Ngigi et al, , 2006Ngigi et al, , 2007Makurira et al, 2007Makurira et al, , 2009Makurira et al, , 2011Rockström et al, 2004;Temesgen et al, 2008Temesgen et al, , 2009Temesgen et al, , 2012Vishnudas et al, 2012) would not only improve farm productivity by soil and water conservation, but would also increase carbon storage in the soil and enhance carbon sequestration in farming produce. If implemented at global scale, this would reduce carbon emissions substantially.…”

Section: Water Governance: Instruments At the Global Scalementioning

confidence: 99%

Evolving water science in the Anthropocene

Savenije

Hoekstra

Zaag

2014

143

Abstract. This paper reviews the changing relation between human beings and water since the Industrial Revolution, a period that has been called the Anthropocene because of the unprecedented scale at which humans have altered the planet during this time. We show how the rapidly changing world urges us to continuously improve our understanding of the complex interactions between humans and the water system. The paper starts by demonstrating that hydrology and the science of managing water resources have played key roles in human and economic development throughout history; yet these roles have often been marginalised or obscured. Knowledge of hydrology and water resources engineering and management helped to transform the landscape, and thus also the very hydrology within catchments itself. It is only fairly recent that water experts have become conscious of such mechanisms, exemplified by several concepts that try to incorporate them -integrated water resources management, eco-hydrology, socio-hydrology. We have reached a stage at which a more systemic understanding of scale interdependencies can inform the sustainable governance of water systems, using new concepts like precipitation sheds, virtual water transfers, water footprints, and water value flow.