Downstream Water Availability Research Articles

Quantitative simulation tools to analyze up- and downstream interactions of soil and water conservation measures: Supporting policy making in the Green Water Credits program of Kenya

Upstream soil and water conservation measures in catchments can have positive impact both upstream in terms of less erosion and higher crop yields, but also downstream by less sediment flow into reservoirs and increased groundwater recharge. Green Water Credits (GWC) schemes are being developed to encourage upstream farmers to invest in soil and water conservation practices which will positively effect upstream and downstream water availability. Quantitative information on water and sediment fluxes is crucial as a basis for such financial schemes. A pilot design project in the large and strategically important Upper-Tana Basin in Kenya has the objective to develop a methodological framework for this purpose. The essence of the methodology is the integration and use of a collection of public domain tools and datasets: the so-called Green water and Blue water Assessment Toolkit (GBAT). This toolkit was applied in order to study different options to implement GWC in agricultural rainfed land for the pilot study. Impact of vegetative contour strips, mulching, and tied ridges were determined for: (i) three upstream key indicators: soil loss, crop transpiration and soil evaporation, and (ii) two downstream indicators: sediment inflow in reservoirs and groundwater recharge. All effects were compared with a baseline scenario of average conditions. Thus, not only actual land management was considered but also potential benefits of changed land use practices. Results of the simulations indicate that especially applying contour strips or tied ridges significantly reduces soil losses and increases groundwater recharge in the catchment. The model was used to build spatial expressions of the proposed management practices in order to assess their effectiveness. The developed procedure allows exploring the effects of soil conservation measures in a catchment to support the implementation of GWC.

Impact of rainwater harvesting on water resources of the modder river basin, central region of South Africa

Along the path of water flowing in a river basin are many water-related human interventions that modify the natural systems. Rainwater harvesting is one such intervention that involves collecting and use of surface runoff for different purpose in the upstream catchment. Increased water consumption at upstream level is an issue of concern for downstream water availability to sustain ecosystem services. The upper Modder River basin, located in a semi arid region in the central South Africa, is experiencing intermittent droughts causing water shortages for agriculture, livestock and domestic uses. To address this problem a technique was developed for small scale farmers with the objective of collecting and concentrating of rainwater for crop production. However, the hydrological impact of a wider adoption of this technique by farmers has not been well quantified. In this regard, the SWAT hydrological model was used to simulate potential hydrological impact of such practices. The scenarios studied were: (1) baseline scenario, based on the actual land use of 2000, which is dominated by pasture (combination of natural and some improved grass lands) (PAST); (2) partial conversion of actual land use 2000 (PAST) to conventional agriculture (Agri-CON); and (3) partial conversion of actual land use 2000 (PAST) to in-field rainwater harvesting which was aimed at improving the precipitation use efficiency (Agri-IRWH).SWAT was calibrated using both observed daily as well as monthly streamflow data of a sub-catchment (419km2) in the study area. SWAT performed well in simulating the streamflow giving Nash and Sutcliffe efficiency of 0.57 for the monthly streamflow calibration. The simulated water balance results showed that the highest peak mean monthly direct flow was obtained under the Agri-CON land use (18mm), followed by PAST (12mm) and Agri-IRWH land use (9mm). These were 19%, 13% and 11% of the mean annual rainfall, respectively. The Agri-IRWH scenario reduced the annual direct flow by 32% compared to Agri-CON which is significant at p<0.02 level. On the other hand it was found that the Agri-IRWH contributed to more groundwater recharge (40mm/year) compared to PAST (32mm/year) and Agri-CON (19mm/year) scenarios. Although there was observable impact of the rainwater harvesting technique on the water yield when considered on a monthly time frame, the overall result suggests that the annual water yield of one of the upper Modder River Basin quaternary catchment will not be adversely affected by the Agri-IRWH land use scenario despite its surface runoff capture design.

On the forest cover–water yield debate: from demand‐ to supply‐side thinking

Several major articles from the past decade and beyond conclude the impact of reforestation or afforestation on water yield is negative: additional forest cover will reduce and removing forests will raise downstream water availability. A second group of authors argue the opposite: planting additional forests should raise downstream water availability and intensify the hydrologic cycle. Obtaining supporting evidence for this second group of authors has been more difficult due to the larger scales at which the positive effects of forests on the water cycle may be seen. We argue that forest cover is inextricably linked to precipitation. Forest-driven evapotranspiration removed from a particular catchment contributes to the availability of atmospheric moisture vapor and its cross-continental transport, raising the likelihood of precipitation events and increasing water yield, in particular in continental interiors more distant from oceans. Seasonal relationships heighten the importance of this phenomenon. We review the arguments from different scales and perspectives. This clarifies the generally beneficial relationship between forest cover and the intensity of the hydrologic cycle. While evidence supports both sides of the argument – trees can reduce runoff at the small catchment scale – at larger scales, trees are more clearly linked to increased precipitation and water availability. Progressive deforestation, land conversion from forest to agriculture and urbanization have potentially negative consequences for global precipitation, prompting us to think of forest ecosystems as global public goods. Policy-making attempts to measure product water footprints, estimate the value of ecosystem services, promote afforestation, develop drought mitigation strategies and otherwise manage land use must consider the linkage of forests to the supply of precipitation.

Analysis of seasonal land use in Usangu wetlands, Tanzania: an object-oriented technique for multi-temporal analysis with high-resolution data

A multi-temporal analysis of Landsat panchromatic data was applied to the Usanga wetlands in Tanzania, where there are concerns that irrigated agricultural development is reducing downstream water availability. As there are many small fields, and even smaller areas of continuous vegetable cultivation throughout the year, 15-m resolution multi-temporal Landsat Enhanced Thematic Mapper Plus (ETM+) panchromatic data, acquired in the months of March, May and October, were used to extract information on the seasonal land use/cover in order to investigate agricultural water use. Object-oriented analysis was used to interpret these high-resolution data through definition of image segments, which were then classified using a pixel-based unsupervised classification method. This hybrid approach helps to process high-resolution data quickly and efficiently, so as to economically and effectively understand and classify land use/cover in highly heterogeneous and seasonally dynamic tropical conditions, where land units are typically small.

The downstream externalities of harvesting rainwater in semi-arid watersheds: An Indian case study

► We assess the downstream impacts of upstream rainwater harvesting in a water scarce river basin in India. ► Using data on crop water use, rainwater harvesting investments, land use, agricultural prices and production costs we simulate the impacts on cropping patterns and assess the change in agricultural value produced. ► The results indicate that upstream benefits are insufficient to compensate downstream losses and pay back investment costs when benefits are limited to a re-allocation of irrigation water. Water-related investment projects affect downstream water availability, and therefore should account for these externalities. Few projects do, however, owing to lack of awareness, lack of data and difficulty in linking upstream investments to downstream effects. This article assesses the downstream impacts of rainwater harvesting in a semi-arid basin in Southern India, focusing on the trade-offs that arise when crop water use is re-allocated from a downstream surface water irrigation system to groundwater irrigated agriculture upstream. The results indicate that the downstream impacts are considerable and that net benefits are insufficient to pay back investment costs. Further research is required to reduce the uncertainties in the water balance of irrigation systems at basin level, to account for the inter-annual variability of crop water availability and to elaborate the wider welfare effects.

Effects of landuse change on the hydrologic regime of the Mae Chaem river basin, NW Thailand

Summary Conflicts between upland shifting cultivation, upland commercial crops, and lowland irrigated agriculture cause water resource tension in the Mae Chaem watershed in Chiang Mai, Thailand. In this paper, we assess hydrologic regimes of the Mae Chaem River with landuse change. Three plausible future forest-to-crop expansion scenarios and a scenario of crop-to-forest reversal were developed based on the landcover transition from 1989 to 2000, with emphasis on influences of elevation bands and irrigation diversion. Basin hydrologic responses were simulated using the Distributed Hydrology Soil Vegetation Model (DHSVM). Meteorological data from six weather stations inside and adjacent to the Mae Chaem watershed during the period 1993–2000 were the climate inputs. Computed stream flow was compared to observed discharge at Ban Mae Mu gauge on Mae Mu river, Ban Mae Suk gauge on Mae Suk river, and at Kaeng Ob Luang, located downstream from the district town in Mae Chaem. With current assumptions, expansion of highland crop fields led to slightly higher regulated annual and wet-season water yields compared to similar expansion in the lowland–midland zone. Actual downstream water availability was sensitive to irrigation diversion. This modeling approach can be a useful tool for water allocation for small watersheds undergoing rapid commercialization, because it alerts land managers to the potential range of water supply in wet and dry seasons, and provides information on spatial distribution of basin hydrologic components.

Integrated assessment modelling for water resource allocation and management: A generalised conceptual framework

Nodal network approaches are a common framework for considering water allocation in river basins. In this type of model framework, a river basin is represented as a series of nodes, where nodes generally represent key points of extraction or instream use. When considering water allocation, agricultural production and other water use decisions generally interact with the stream system in two ways: they can affect the generation of runoff and thus the volume of water reaching the stream; or, they may involve direct extraction or use of water once it has reached the stream. Models are generally required to consider the influence of these decisions on flows and downstream water availability, as well as the influence of flows on the productive, passive use and environmental values of water. This paper provides a generalised conceptual framework for considering these types of interactions and their representation in integrated water allocation models. Applications of this framework to three very different case studies are outlined.