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A Google Earth-GIS based approach to examine the potential of the current rainwater harvesting practices to meet water demands in Mityana district, Uganda [1]

['Jamiat Nanteza', 'Department Of Geography', 'Geo-Informatics', 'Climatic Sciences', 'Makerere University', 'Kampala', 'Brian Thomas', 'School Of Civil Engineering', 'Geosciences', 'Newcastle University']

Date: 2022-12 

Abstract Rainwater harvesting (RWH) has become an integral part of global efforts to improve water access. Despite the increasing adoption of RWH in Uganda, there remains a significant knowledge gap in the assessment of RWH systems to meet water demands. In this study, a simplified methodology to estimate rainwater harvesting potential (RWHP) as a function of mean seasonal rainfall and rooftop area, generated using Google Earth and GIS tools is applied. Desired tank storage (DTS) capacities based on user population, demand and dry period lengths, were compared with RWHP to assess whether rooftop areas and tank storage can sustainably supply water for use during the March—May (MAM) and September-November (SON) 90-day dry periods, for three demand levels (i.e. for drinking and cooking (15 litres per capita per day (l/c/d)); for drinking, cooking and hand washing (20 l/c/d); and for drinking, cooking, hand washing, bathing and laundry (50 l/c/d)). Our findings document minimum catchment areas of 60m2 to have rainwater harvesting potential that can sustain households for 90-day dry periods for all three demand levels. However, considering their storage capacities, 25%, 48% and 97% of the existing RWHTs (with storage capacities below 8,000, 10,000 and 20,000 litres respectively) are unable to meet the demand of 15 l/c/d, 20 l/c/d and 50 l/c/d respectively for a 90-day dry period. The results document that the existing storage systems are under-sized for estimated water use under 50 l/c/d demand scenarios. Costs of between 2,000,000–4,500,000 Ugandan shillings (~ 600–1, 250 USD) would be needed to increase existing tank capacities to meet the 50 l/c/d demands for a 90-day dry period. These findings document onerous financial costs to achieve rainwater harvesting potential, meaning that households in Mityana district may have to resort to other sources of water during times of shortage.

Citation: Nanteza J, Thomas B, Kisembe J, Nakabugo R, Mukwaya PI, Rodell M (2022) A Google Earth-GIS based approach to examine the potential of the current rainwater harvesting practices to meet water demands in Mityana district, Uganda. PLOS Water 1(11): e0000045. https://doi.org/10.1371/journal.pwat.0000045 Editor: M. J. M. Cheema, PMAS Arid Agriculture University: University of Arid Agriculture, PAKISTAN Received: March 3, 2022; Accepted: October 26, 2022; Published: November 23, 2022 This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability: Original data on rainwater harvesting tanks used in this study is from the Ministry of Water and Environment Uganda and requires a user to seek permission before access (http://wsdb.mwe.go.ug/). The contact for the ministry is Tel: +256 417 889 400 Email: mwe@mwe.go.ug. Funding: This study was supported by the Partnership for Enhanced Engagement in Research (PEER) grant (NAS Subaward number: 2000007513,Sponsor Grant Award Number: AID-OAA-A-11-00012) awarded to the first author JN by the National Academy of Sciences (NAS) on behalf of the USAID (https://sites.nationalacademies.org/PGA/PEER/index.htm) for the period 2016 -2020. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

1. Introduction The dependence on water for drinking, basic sanitation and hygiene increases in relation to the growing population, which when combined with the increasingly uncertain and variable climate, can rapidly degrade water availability. Rural communities in Sub-Saharan Africa that depend on groundwater for drinking water often face water shortage challenges during dry seasons when groundwater levels decline, resulting in seasonal well failure [1], effectively eliminating the primary household water source. Households resort to securing domestic water needs from secondary water sources, often surface water from nearby streams [2] that are typically of poor water quality and may increase time required to fetch daily water needs. Rainwater has long been recognized as a strategic renewable water source [3, 4] which, if efficiently harvested and stored, can augment groundwater and surface water shortages during periods of insufficient water availability [5], boosting water reliability to fulfill water demands. Adoption of rainwater harvesting has increased worldwide e.g., in India [6], China [7], South Africa [8], Nigeria [9]. In particular, the recognition of increased reliability for freshwater availability when augmenting water supplies using rainwater harvesting has resulted in rapid development in areas with seasonally distributed rainfall, common in Sub-Saharan Africa. Rainwater harvesting (RWH) systems collect and store rainfall or rainfall-induced runoff [10, 11], through technologies such as tanks or gullies. RWH provides positive benefits given a myriad of uses. For instance, RWH has been shown to buffer the impacts of increased variability in timing and amount of rainfall on rain-fed agricultural production and livestock [11]. Rainwater harvesting practices improved farmer incomes [12] e.g., in Tanzania, where increased gross margins and returns to labor in onion and maize growing have been documented. In both rural and urban communities, RWH systems are used to ensure continual access to portable water during periods of reduced water availability [13], fulfilling vital domestic water and basic hygiene demands during dry periods in Sub-Saharan Africa. For instance, in Abeokuta, Nigeria, harvested rainwater during the peak rainfall months of June, September and October was sufficient to satisfy household water demand for water closet (WC) flushing and laundry during the dry months of November-February [14]. In Uganda, the increased adoption of RWH systems has spurred numerous studies to assess system benefits. Studies examining the impact of rainwater harvesting on agricultural production/food security [15, 16] and climate change adaptation [17] found that rooftop rainwater harvesting has the potential to satisfy domestic needs and support agricultural production during dry periods, leading to persistent production of crops (e.g., vegetables, maize and potatoes) and livestock and thus sustained volumes of cash and food crops. Studies e.g., [18] examining the future impact of rainwater harvesting on water security under changing climates found that water savings and security would reduce in December—February and March—May seasons and increase in June—August and September—November seasons, a phenomena that calls for measures by households to harness the increased water savings in JJA and SON to cater for the predicted reduction in water savings in MAM and DJF seasons. A key issue in recognizing the benefit of RWH is adoption of RWH practices [19], where hesitation is often due to cost [20–23]. Broad recommendations for tank sizing by Uganda’s Ministry of Water and Environment (https://www.mwe.go.ug/library/rain-water-harvesting-handbook, August, 18 2021) do not account for reliability to meet desired water demands given a range of water uses and may limit RWH adoption given price [23]. Additional issues with RWH adoption include water quality [24, 25], documented to be attributed to maintenance and gender perspectives [26]. A complicating factor in evaluating RWH potential is the local variability in precipitation [27], especially where rainwater harvesting generalization remains elusive, such as the mountainous regions in eastern Uganda [13]. Although these studies have provided strong evidence regarding the utility of RWH to benefit water supply resilience in Uganda, there remains a knowledge gap to quantify RWH storage potential to avoid over/under catch and minimize economic loss. A significant limitation in rainwater harvesting development in Uganda, as is true in many developing countries, is a lack of a generalization framework to guide optimum storage size in relation to household characteristics (e.g., number of family members, household water demands) and climate (e.g., seasonal variability in precipitation). A complicating factor in the development of a generalization framework is the capitalistic nature of RWH production, where manufacturers operate for economic gain using pre-fabricated RWH storage options without consideration of the design parameters for optimum storage. Our experience in Uganda also confirms that household decisions to purchase/construct RWH technologies largely depend on the economic capability and perceptions of purchasers, with no consideration of the design parameters for optimum storage and metrics of reliability during periods with no filling due to a lack of precipitation. As a result, RWH systems are often incorrectly sized due to the pre-fabrication sizing of storage systems, leading to over (under) utilization of the RWH potential. Inefficiently sized RWH systems represent economic losses, either due to unnecessary construction and material costs for oversized systems or opportunity cost for undersized systems. To determine optimal RWH potential storage, at a minimum, an estimate of rooftop/catchment area is required. Rooftop/catchment area data is limited in Uganda, leading to simplifications, such as the use of a single rooftop area in many previous studies (e.g., [13, 15]). The ill-advised application of a single rooftop area value can be overcome with Google Earth and GIS technologies, whereby site-specific rooftop area estimates may be extracted (e.g., [28–33]). Roof area estimation approaches have, to date, been applied over Africa (e.g., [34]), Asia (e.g., [33, 35, 36]), and Europe (e.g., [37]), providing a framework to quantify RWH potential over any region, including Uganda. This study applies a Google Earth—GIS based approach to examine the rainwater harvesting system potential to meet water demands in Uganda. In this study, we restrict our evaluation to existing (i.e., installed and operational) rooftop rainwater harvesting at household level, where rainwater on rooftops is conveyed and stored to a centralized tank system for domestic uses (e.g., drinking, cooking and sanitation and hygiene). The objectives of the study are to 1) estimate rooftop areas associated with existing RWH storage tanks using Google Earth Pro and GIS, 2) estimate the potential volume of rainwater that can be harvested given estimated rooftop areas, 3) evaluate the over (under) utilization of existing rainwater harvesting tank storages to fulfill desired water demands during dry seasons, and 4) quantitatively express the economic implications of over (under) utilization. Our study is conducted over the Mityana district in Uganda (Fig 1) given that it is one of the districts in Uganda where rainwater harvesting has been actively promoted by individuals and development partners (e.g., Uganda Community Based Association for women and Children’s welfare (UCOBAC)). PPT PowerPoint slide

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TIFF original image Download: Fig 1. A map of Mityana district showing the location of rainwater harvesting tanks (base map sourced from the Uganda Bureau of Statistics (UBOS) from the link A map of Mityana district showing the location of rainwater harvesting tanks (base map sourced from the Uganda Bureau of Statistics (UBOS) from the link https://ubos.maps.arcgis.com/home/item.html?id=4e92034071494dffb239a219449fd2c1 , accessed January, 15 2021). https://doi.org/10.1371/journal.pwat.0000045.g001

4. Conclusions and recommendations Our results reveal important reflections for rainwater harvesting in Mityana district and for Uganda at large. Rooftops in Mityana district range between 15-750m2 with an average rooftop area of 170 m2, similar to rooftop areas used by other studies over Uganda (e.g., [13, 15]). Results demonstrate that majority of rooftops would reliably collect enough water to meet water demands for drinking, hand washing and cooking (i.e., 15 and 20 l/c/d) when accounting for rainfall season precipitation (MAM and SON). However, under a water demand scenario of 50 l/c/d, suitable to fulfill basic needs plus laundry and bathing, rooftop areas of 60–70 m2 could reliably collect enough water to fulfill demands for the 90-day dry periods. This result is in close range to earlier findings [18] that rooftops of minimum size 50 m2 are sufficient to collect enough water to improve water security during dry periods. On ground, evidence indicates that households who venture into rainwater harvesting install systems without considering rooftop area and rainfall characteristics over their catchment area. This practice leads to underutilization or overutilization of the rainwater harvesting potential for different households given a range of water demand levels. In Mityana, a significant number (i.e. 97%) of existing rain water harvesting systems are undersized especially for high water demands 50 l/c/d for the 90-day dry period. This result is similar to published findings [58] which demonstrated that at high demand levels, such as 50 litres per person per day, domestic rainwater harvesting in parts of Africa would rarely meet all household water demands. Future changes in precipitation patterns [59] may exacerbate rainwater harvesting reliability, potentially compromising water demand requirements including basic hygiene. In order to increase the RWH’s capacity to meet the high water demands of 50 l/c/d within Mityana district, additional costs of at least 2,000,000 UGX (600 USD) are required. For some households, such costs would put pressure on their economic means, and thus restrict their ability of to increase storage capacity, requiring these households to resort to other sources of water during times of shortage. It is worth noting that information about the sole purpose of these tanks was not available for this study. For if the sole purpose is to store water for drinking and cooking only, then the majority of the tanks would be sufficient for a 90-day dry season (i.e. MAM and SON seasons). However, if the purpose is to cater for drinking, cooking, bathing and laundry, most private tank owners would have to pay extra money in the range 600–1,250 USD to meet the 50 l/c/d demand for a 90-day dry period. The results reveal that RWH systems with storage capacities of 25, 000 litres and more maybe more viable in meeting the 50 l/c/d demand in Mityana district for one dry season. This study however did not consider other water sources that the different households use to supplement their water demands, information that would be significant in further shaping the conclusions from this study. In addition, we assumed that the demand is constant per capita per day, however water use tends to vary from day to day. We thus recommend that to fully understand the capacity of rainwater harvesting in addressing water needs, more work be done in understanding the sole purpose of the private water harvesting tanks, the dynamics of daily water use, the alternative water sources and their water use purposes. In addition, the study used a simplified method that uses seasonal precipitation in computing water harvesting potential. Much as this method is recommended and commonly used for domestic-rainwater harvesting systems where demand is regular, it doesn’t cater for the rainfall variability over time, which parameter affects RWH system performance [60]. It is also shown that simplified approaches generate large tank sizes compared to other methods like models [61]. We therefore recommend that future studies apply more advanced methods such as regression models [60] to estimate optimum RWH systems storage capacities while considering daily climate variability. Nonetheless, our findings indicate that a significant number of the existing RWH systems in Mityana district are not correctly sized. Thus, numerous RWH systems would fail to reliably provide sufficient water for drinking, laundry and hygiene during the dry seasons (Fig 8C). Inaccurate and ineffective tank capacity sizing reflects an opportunity cost; whereby undersized systems may require households to secure water for basic needs from unreliable and unsuitable sources. However, if RWHs are correctly sized, our findings suggest that adequate volumes of water could be captured and stored (Fig 7A and 7B). Therefore, this study recommends that measures to improve access to clean water through rainwater harvesting consider correct sizing of RWHs.

Acknowledgments The support of Makerere University, Department of Geography, Geo-Informatics and Climatic Sciences is gratefully acknowledged. This study was also made possible using freely available water supply data from Ministry of Water and Environment, Uganda available at (http://wsdb.mwe.go.ug/).

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