Water Harvesting for Groundwater Recharge

1. Water Harvesting for Groundwater Recharge Is it effective?

2. INTRODUCTIOIN Water scarcity is becoming an increasing problem worldwide. • 35% of global land surface is semi-arid • In 1997 it was reported that 80% of countries suffered from serious water shortage, which encompasses 40% of the worlds population

3. Main reason due to a shift in water management across the world. • Individuals/communities have given over the role of water management to the State/Government. • Increasing exploitation of rivers and groundwater – these are now the key sources of water supply across the world. However, it is generally believed that enough precipitation falls on the worlds land surfaces to supply the global population. • Use of water harvesting to capture the water that falls before it gets chance to be evaporated

4. WATER HARVESTING • Term was 1st used by the Australian HJ Geddes to denote: ‘the collection and storage of any farm waters, either runoff or creek flow, for irrigation use.’ • Idea has been around since early civilization, beginning about 4000years ago in the Bronze Age • Desert dwellers smoothed hillsides to increase runoff and built ditches to collect the water and convey it to lower lying fields • Also included collection from rooftops and courtyards

6. There has been an increasing interest in the redevelopment and implementation of water harvesting systems, especially of water harvesting for groundwater recharge Groundwater is a more favourable source of water for several reasons • It provides ready made storage reservoirs • It helps reduce the problems of evaporation • It protects against pollution due to the filtering action of the aquifer • It helps keep saline waters from intruding • It helps convert/dispose of floodwaters • It helps to prevent subsidence of the ground above the depleted aquifer

7. WATER HARVETSING TECHNIQUES FOR GROUNDWATER RECHARGE Artificial recharge of groundwater as define by UNEP is ‘The planned human activity of augmenting the amount of groundwater available through the works designed to increase the natural replenishment or percolation of surface waters into the groundwater system, resulting in a corresponding increase the amount of groundwater available for abstraction.’

8. 2 main techniques have been described for increasing groundwater recharge in arid areas • The planting of trees • Estimates have been suggested that the planting of trees can increase precipitation by over 300% and therefore more water is available for recharge to the aquifer • The construction of water harvesting devices • Include the use of both floodwater and rainfall • Increasing surface runoff into storage/collection areas by decreasing infiltration • Increasing infiltration into the aquifer along the route of travel of surface water • Use of engineering structures to collect and pump the water into the deep aquifer

9. Floodwater harvesting for groundwater recharge There are several methods by which floodwater harvesting can be carried out for aquifer recharge • Check dams • Percolation tanks • Groundwater dams These are amongst the cheapest and most widely used methods for groundwater recharge for 2 main reasons • Relatively low construction costs • Easy to operate and maintain

10. Check dams • Temporary structures constructed from locally available material such as brushwood, loose rock or woven wire • Aim to impede the soil and water removed from the catchment – the impeded water collects behind the dam and infiltrates the soil, recharging the aquifer • Cheap to construct costing approx. US $200-400 depending on the material used, size of the gully and the height of the dam • Life span of 2-5 years • More permanent dams constructed using stones, brick and cement, but costs increase to US $1000-2000

11. Percolation tanks This is a dam built on permeable ground so that floodwater is held back long enough to percolate into the ground • More permanent and larger than check dams • Constructed by excavating a depression to form a small reservoir or by constructing an embankment in a natural ravine or gully • Cost is estimated to be approx. US $5,000-10,000 • Capacity varies from 10,000-15,000m3 • 2-3 filling expected per wet season (30,000-45,000m3) • Enough to irrigate 4-6 ha of irrigated dry crop (maize) and 2-3 ha of paddy crop

13. Groundwater dams • Structures that intercept or obstruct the natural flow of groundwater • Often built within river beds to obstruct and detain groundwater flow so as to sustain the storage capacity of the aquifer and meet demands during high periods • Used in many areas such as India, Africa and Brazil • 2 main types • Sub-surface dam • Sand storage/silt trapping dams

14. Sub-surface dam • Constructed within the aquifer itself • Dam reduces variation of the level of the groundwater upstream

15. Sand storage dam • Constructed above ground • Sand and silt particles transported in floodwater and get deposited behind the dam • Constructed in layers to trap the sand with each flood

16. Important considerations • Material used must be impermeable for the permanent structures • Dam must be strong enough to withstand the build up of the sediment behind it • Aquifer should be confined beneath to prevent seepage • Bitumen/plastic lining

17. Rainwater harvesting for groundwater recharge • More related to the ‘micro-catchment’ • Roofs • Artificial surfaces at ground level e.g. courtyards • Slopes less than 50-150m in length • Contour trenches • Excavations made parallel to contours on slopes • Rainwater ponds and infiltrate • Helps to prevent eroding soil migrating down-slope • Gully plugs • Stone barriers built across gullies and deep rills • Traps sediment eroded from higher up slope and impound runoff encourage encouraging infiltration

18. Rooftop harvesting • Can be adopted by individuals • Relatively easy to construct, operate and maintain • Several techniques in use • Abandoned dug well • Abandoned/running hand pump • Gravity head recharge • Recharge pit

23. Recharge by injection well • Direct sub-surface recharge technique conveying water directly into the aquifer • Can be used in conjunction with rooftop harvesting • Concerns that groundwater may get contaminated • Potential for clogging from suspended solid, biological activity or chemical impurities • Expensive to construct and harder to maintain and operate than the other methods

25. Is water harvesting for groundwater recharge effective? Important initial considerations • Only suitable for areas where aquifers exist • Simpler with unconfined aquifers • Thorough survey of the area is necessary • Climatic records (ppt, humidity, evaporation rates) • Topographical maps (drainage networks, ephemeral streams) • Soil data (type, distribution and thickness) • Distribution of rock types, esp. surface features • Definition of pore networks • Recognition of recharge/discharge areas, direction of groundwater flow

26. Once a suitable site has been found • Essential to involve the local community Once the structure has been built • Important that the system is managed properly • Unnecessary consumption of water • Groups of locals need to be put in charge of the system • Ensure adequate distribution amongst the villagers • Poor farmers/owners of small farms

27. Potential problems • Contamination from direct injection • Contaminate water already stored in the aquifer • Lower the value of the water (drinking water) • Water quality problems from rooftop harvesting • Dust/faeces washed off the roof • Large evaporative losses • Silting • politics

28. SUMMARY • Many potential problems • If planned and managed properly can be a great success • More effective for shallow aquifers • More accessible for farmers • Cheaper to extract the water • Recharged more effectively with little high-tech equipment