How do we ascertain the percentage of water recharge available through sub-surface wells?

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Dear Sir/Madam,

I am a practitioner working on watershed development. I work in Odisha which is a high rainfall area (1200-1400 mm annual rainfall) and in areas where the main source of irrigation is open wells. There are very few bore/tube wells.

I seek your assistance is suggesting papers/sharing your thoughts on the following questions:

1. During monsoon what% of rainfall is retained in the subsurface soil, which is available through shallow wells?

2. How long the subsurface water available in shallow wells after monsoon?

3. If the water has been withdrawal from shallow wells for irrigation, how long it takes to recover? (Apart from the parameters of soil texture and geology)

4. If a borewells has been pumped, does it also reduce the water in shallow wells. Are there any thumb rules guiding this?

Your suggestions would be very helpful.

Regards

Subrat

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3 Answers

  1. Thanks Andrew & Randall for your responses. The thumb rules Andy shared are useful. 

    The information I had sought was to evolve parameters for crop-water budgeting during non-monsoon season (cropping is dependent on irrigation). As the area has very little ground water development, most of the water is accessed using shallow wells.

    Ascertaining water availability in the shallow wells has been tricky - as water yields in wells vary widely - influenced by geology.

    We have developed a GIS tool called the Composite Land Assessment and Restoration Tool (CLART) - developed by overlaying geology, linaments, drainage pattern, slope and land use to come up with a recharge map - to show the recharge potential. While the tool helps ascertain recharge % based on recharge potential, ascertaining the % available in shallow wells has been difficult.

    Are there specific research papers that provide algorithms to calculate the same

     

    Regards 

  2. Subrat

    The answers to your question will be very much determined by the geology. If you are in an area where mostly farmers are using dug wells, then I am guessing you are on metamorphic basement rocks, with water held in the weathered zone.

    1. The amount of water that infiltrates in the monsoon is very much affected by soil type and topography, but for a very very approximate initial guess, you might assume that 10% of the rainfall recharges groundwater - so in your case 140 mm (it could be higher or lower) . Now this water that recharges is not all available for shallow wells - some may flow out of the zone, or infiltrate deeper, some may support dry season water demands of trees and vegetation, or provide baseflow in streams. In India a figure of 85% of recharge being available for abstraction is sometimes used, but many authors would consider this too high. For an initial guess, assume 50%  so an available water of about 70mm per year.

    2. How long water is available post monsoon is again very much about geology and topography, but this time will also depend on how wells are dug and how much water farmers use.  As water naturally evaporates and farmers pump water the level in the aquifer will fall. If wells are shallow and pumping rates high, this may happen quickly - or with deep wells and little pumping, it may be that water lasts all year round. 

    3. Again - you can't ignore the geology and soil texture.  If we assume that the weathered soils have 10% porosity, 70 mm of  infiltrated rainfall will raise the water table by 700 mm. So if the water table had fallen by 2 metres over a wide region, it would take nearly 3 years of average rainfall with no pumping to recover.

    4.  In most cases if the geology is weathered metamorphic rocks pumping borewells will certainly affect shallow wells. The borewells will gather water from fractures in the rock, but those fractures will get their water from the same rainfall as the shallow wells, so the overall budget of water is just the same.  It is possible (usually in sedimentary or volcanic rock provinces) that the deep borewells draw water from rocks where water travels horizontally (e.g. from a hilly area to the farmland), but you can only tell this with careful geological investigation.

    Now all of this is speculative.  The calculation of recharge depends on the soil and geology, and I haven't accounted for irrigation water losses flowing back to the aquifer. The connectivity between shallow and deep aquifers is equally speculative.

    In your position I would probably seek some local geological advice (from the CGWB or state authority) - but also try and get a good inventory of how many wells there are, and how much water is pumped. Monthly monitoring of water levels in wells can help you understand how the system is working, by observing how levels rise in the monsoon and fall in the dry season.

    Andy

  3. We have analyzed some of this through modeling and wellbore surveys such as flow meters under pumped and unpumped conditions. We have many papers where we have looked at deeper wellbores. Shallower wells are commonly disconnected from deeper pumping unless the deeper wells are also screened in the shallower zones and are draining the shallower aquifers through well bore flow

    feel free to contact us for additional information

    1 Comment

    1. In an area with less hydrological stress, depending on its permeability, aquifers can gain water at a rate of 50 feet per year. They have both recharge and discharge zones. A recharge zone usually occurs at a high elevation where rain, snow melt, lake or river water seeps into the ground to replenish the aquifer. The percentage can be obtained using MODFLOW after acquiring some input parameters