Evaluating Rainwater Harvesting Systems in Arid and Semi-Arid Regions

Abstract

Rainwater harvesting (RWH) is an ancient traditional technology practiced in many parts of the world, especially in arid and semi-arid regions (ASARs). ASARs represent 40% of the earth’s land surface and are characterised by low average annual rainfall and uneven temporal and spatial distributions of that rainfall. These climatic characteristics indicate that using the limited amount of rainfall available as efficiently as possible is important. One method for doing this is to collect and use surface runoff (water harvesting). The inhabitants of ASARs have developed several RWH techniques for increasing the availability of water and thereby coping with water shortages. Over the past century, access to water for agriculture and domestic use has become worse because of increasing population, higher levels of human activity and the impacts of climate change. Climate change is a very serious problem and has become a major global issue, especially in developing countries which are severely affected by its impacts. RWH is seen as an important mitigation strategy to the impact of climate change on water availability in ASARs. A robust methodology is therefore needed to assess the potential for rainwater harvesting and identify areas that are suitable for these techniques. Also further knowledge regarding the impact of climate change on the functioning of RWH in the future is needed to assess their ability to meet future water requirements. A general overview of the history of RWH techniques, a review of the literature concerning these techniques and brief descriptions of the available models are presented in Chapter 1. The motivation for using the results of general circulation models (GCM) in the design of RWH structures is also given. An inventory of the main methods and criteria developed in ASARs during the last three decades and a general method for selecting suitable RWH sites in ASARs are presented in Chapter 2. Four main methodologies of site selection were categorised based on 48 studies published in scientific journals, reports of international organisations, or sources of information obtained from practitioners. The most suitable method for application in a particular case was highly dependent on the main objectives and needs of the project (e.g. flexible, widely applicable, efficient and accurate) and on the quality, availability and reliability of the data. The methods were diverse, ranging from those based only on biophysical criteria to more integrated approaches that include socioeconomic criteria, especially after 2000. Three main sets of criteria for selecting RWH locations were 184 identified, and the main characteristics of the most common RWH techniques used in ASARs are presented. This study identified slope, land use/cover, soil type, rainfall, distance to settlements/streams and cost as the most important biophysical and socioeconomic criteria for the selection of suitable sites for RWH in ASARs. The most common techniques developed and used in ASARs were also identified: ponds, check dams, terracing, percolation tanks and nala bunds. Our analysis of the strengths and weaknesses of RWH assessment methodologies suggests that the integration of multi-criteria analysis (MCA) with a geographic information system (GIS) is the most advanced approach and provides a rational, objective and unbiased method for identifying suitable sites for RWH. MCA integrated with GIS offers high potential in data-poor regions; GIS-based hydrological modelling is always recommended for data-rich regions. The research project started with a case study on the potential for RWH in Iraq (Chapter 3). For safety reasons, the method for selection of suitable RWH locations was restricted to factors for which GIS data were available. Potential RWH sites in wadi Horan, located in the western desert of Iraq, were identified using a GIS-based suitability model. The suitability model combined different biophysical criteria: slope, runoff depth, land use, soil texture and stream order. Areas suitable for dams were identified by reclassifying these layers and combining them using the raster calculator tool in the spatial analyst module of ArcGIS 10.2. Each criterion was clipped to the study area, reclassified to numeric values and assigned suitability rankings for dams. The selected sites were then assessed by the other criteria to identify the best sites for RWH structures (dams). A suitable site for a dam is a place where a wide valley with high walls leads to a narrow canyon with tenacious walls. Such sites minimise dam dimensions and costs, but steep valley slopes should be given a low priority, because dams at such sites are rarely economical. 39 potential sites were identified based on the visual interpretation of satellite images and an analysis of large-scale cartography. Each potential dam site was further analysed by calculating characteristics such as the available storage area and the required length and height of the dam. The present study found that ArcGIS was a very useful tool for integrating diverse information to find suitable sites for RWH. ArcGIS is a flexible, time-saving and cost effective tool for screening large areas for their suitability to be used for RWH intervention. Fieldwork should be carried out on the selected sites to ensure that they do not conflict with other land uses in the area that are not identified with the available GIS data. The analysis as presented, however, provides a valuable first screening of large areas English summary 185 and can be easily modified to incorporate other criteria or information with different spatial resolutions. The method for selecting suitable sites for RWH was then further developed into an evaluation and decision support tool (Chapter 4) for assessing the overall performance of existing RWH techniques and the criteria affecting that performance in ASARs. The support tool developed is robust, inexpensive, simple to apply, reliable and easily adaptable to a variety of criteria, RWH techniques and regions. Based on our suggestions in Chapter 2, this methodology integrates engineering, biophysical and socioeconomic criteria using MCA supported by GIS. A comparable scale between criteria was identified before applying the MCA tools due to the variety of measurements and scales for the criteria. The selected criteria were re-classified into five suitability classes, from 1 (very low suitability) to 5 (very high suitability), for assigning scores to the criteria based on discussion and consultation with experienced people and published information. This methodology was tested in the wadi Oum Zessar in southeastern Tunisia by evaluating 58 RWH locations in three main sub-catchments of the watershed. Based on the criteria selected, 65% of the assessed sites scored near 3 (medium suitability), 31% scored near 2 (low suitability) and only 4%, two sites, scored 4 (high suitability). This study indicated that RWH with low suitability was associated with poor engineering design, lack of proper maintenance and the high cost of water storage. The criteria assessments indicated that rainfall had no substantial impact on the overall suitability between sites in our case study but could be very important for comparisons between sites in larger areas with large differences in rainfall. Our study also found that socioeconomics played an important role in RWH performance and was a very important criterion for improving current RWH effectiveness and planning future structures. Our methodology clearly identified the criteria that should be addressed to improve the performance of, for example, RWH structural design and storage capacity. Based upon the comparison between our observations and the views of local people and experts, our results effectively represented the real performance of each site—both at an overall level and at the level of individual criteria. This confirms that the methodology developed in this project is a good way to assess the performance of RWH structures. To further investigate and optimise the performance of the RWH systems described in Chapter 4 under various scenarios of design and management, a simple but generally applicable water harvesting model (WHCatch) was developed and is presented in Chapter 5. The model is based on the water balance at a catchment level and can be applied with minimum data. WHCatch was developed as a Visual Basic for Applications 186 macro in a Microsoft Excel workbook and can be used to make all necessary calculations as well as to present the results of the modelling. Using WHCatch the performance of RWH systems in wadi Oum Zessar were evaluated and optimised under different scenarios of design and management (Chapter 6). The changes in the water storage of 25 sub-catchments in three types of years (dry, normal and wet) were calculated from the water balances of the sub-catchments. Two cases were considered for the scenarios. In case 1, no relationship between the water flow of the sub catchments was assumed. In case 2, interaction between the sub-catchments was considered. In case 1, about 28% (wet year) and 8% (normal year) of the sub-catchments were able to meet the water requirements. The complete absence of harvested rainwater (zero) for some sub-catchments, however, indicated the inability of RWH to meet the water requirements due to shortcomings in the engineering design, lack of proper maintenance, site selection, or type of RWH adopted, as shown in Chapter 4. In case 2, about 44, 32 and 16% of all sub-catchments had sufficient water to meet the water requirements in a wet, normal and dry year, respectively. The estimated runoff volumes in case 2 were clearly higher compared to case 1, indicating that a series of connected reservoirs can be more efficient than several unconnected reservoirs in the area. With this information three management scenarios were applied under case 2 conditions to improve the performance of the RWH system and water availability. Broken structures were repaired in management scenario 1, flow directions were changed in scenario 2 and scenarios 1 and 2 were combined in scenario 3. Scenario 3, changing the spillway heights together with the flow directions, had a large impact on the performance of the RWH structures: 92% of all sub-catchments supplied sufficient water, compared to just 44% of the sub-catchments before the changes. This study emphasises the advantages of simulating long-term water balances at the sub-catchment level for improving our understanding of hydrological processes in a RWH system, and provides several solutions for optimising RWH performance in various scenarios. The impact of climate change on existing RWH systems in the Oum Zessar watershed under current and future scenarios of climate was also investigated (Chapter 7). Potential adaptive strategies for optimising RWH effectiveness were estimated based on the predicted climate change. To estimate the impact of climate change on the RWH at the sub-catchment level, precipitation and temperature data were downscaled from general circulation models using a statistical downscaling model (SDSM). Three climatic scenarios, representative concentration pathway (RCP) 2.6, RCP 4.5 and RCP 8.5, were analysed for each 30-year period, i.e. 2011-2040, 2041-2070 and 2071-2100. The downscaled maximum and minimum temperatures clearly indicated an increasing trend in the mean English summary 187 monthly temperature for all three scenarios and all future periods. The generated precipitation tended to decrease the mean annual daily precipitation for the three scenarios in all periods. The application of WHCatch demonstrated that water availability in each sub-catchment would decrease under future conditions for all three scenarios (RCPs 2.6, 4.5 and 8.5) and periods, especially at the end of this century. It also indicated that while about 72% of the sub-catchments were able to meet the water requirements of the baseline period, only about 30% would be able to meet the water requirements under any of the future RCP climate scenarios. Here too, the combination of changing both flow direction and the spillway height had a large impact on the performance of the RWH systems. With these changes, the sub-catchments able to meet the baseline water requirements increased to 92% and those able to meet the water requirements in future scenarios increased to 50%. Water management and structural design at the sub-catchment level therefore played a more important role than climate change in the performance of RWH. Chapter 8 presents a synthesis of the major findings of this study and the possible contributions to the scientific efforts for improving the performance of RWH designs under current and future climatic conditions. The implications and recommendations of this study are also presented