Investigating the environmental response to water harvesting structures: a field study in Tanzania

Abstract

Sand dams, a popular water harvesting structure employed by rural communities, capture and store water for use during the dry season in arid and semi-arid regions. Most sand dam research has been performed on the "ideal" sand dam, despite approximately 50 % of sand dams not functioning as intended. This research involves a 1-year long, in-depth field study of three sand dams in Tanzania, one of which is essentially non-functioning. The study investigated a sand dam's impact on macroinvertebrate habitat, vegetation, and streambank erosion and explored a sand dam's water loss mechanisms. Surveys of macroinvertebrate assemblage were performed each season. Vegetation surveys were performed every other month, and erosion was recorded semi-monthly. Water table monitoring wells were installed at each sand dam, and measurements were taken twice a day. The study found that sand dams are too homogeneous to provide the sustenance and refugia macroinvertebrates need at different life stages. The non-functioning sand dam has a thick layer of silt preventing infiltration of rainwater. The functioning sand dams store a significant amount of water, but most is lost to evapotranspiration within a few months of the last rainfall. Unlike the non-functioning sand dam, the functioning sand dams have a positive impact on local vegetation and minimal impact on erosion. Sand dams can increase the water security of a community, but site characteristics and construction methods must be strongly considered to maximize the sand dam's positive impact.

Sand Dams as a Potential Solution to Rural Water Security in Drylands: Existing Research and Future Opportunities

“…While evapotranspiration was not identified as a sensitive factor by either Borst and de Haas (2006) or Quinn et al (2018b), Eisma and Merwade (2020) found that a functioning sand dam loses up to 88% of its stored water via evapotranspiration.…”

“…Calculating the water balance for a sand dam requires collection of various inflow and outflow measurements during an extended field study. As a result, only three studies have collected the necessary data (see Table 1) (Borst and de Haas, 2006;Quinn et al, 2019;Eisma and Merwade, 2020). Of the three water balances, only Quinn et al (2019) and Eisma and Merwade (2020) provide an analysis of intra-seasonal sand dam water storage fluxes.…”

“…Many sandy streambeds store water in drylands, but sand dams scale up this potential by increasing the volume of sand in the streambed. In addition to increasing the potential for water storage, sand dams can also help protect stored water from evaporation (Borst and de Haas, 2006;Quinn et al, 2018b;Eisma and Merwade, 2020), increase the quality of stored water (Quinn et al, 2018a;Graber Neufeld et al, 2020), improve overall ecosystem health (Manzi and Kuria, 2012;Ryan and Elsner, 2016;Eisma and Merwade, 2020), and improve the well-being of the local community (de Bruijn and Rhebergen, 2006;Lasage et al, 2008;Pauw et al, 2008;Maddrell, 2010). Despite numerous positive impacts cited in the literature, some studies note that not all sand dams effectively capture and store water (Van Loon and Droogers, 2006;Nissen-Petersen, 2010;de Trincheria et al, , 2018Eisma and Merwade, 2020;Ngugi et al, 2020).…”

“…For a sand dam to perform optimally, well-sorted sand must be the primary reservoir material. When silt and/or clay settle into the reservoir in large volumes, the ability of the sand dam to capture and store water is significantly reduced Eisma and Merwade, 2020).…”

“…A comprehensive analysis of sand dam literature supports better informed future research and implementation practices. More rigorous scientific work on sand dams and other water harvesting technologies must be encouraged to ensure water project managers are better informed when choosing the FIGURE 2 | Sand dam schematic showing seasonal sand deposition before maturity and two common abstraction methods (adapted from Eisma and Merwade, 2020). most appropriate technology for their target community.…”