How can I calculate multi-refill in water harvesting structures because of monsoon rains?

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We are working on issues on water harvesting with a metrics of water conserved through multiple refills.

Request experts in the field to kindly let me know of the methods and publications if any on the subject. You may also lead me to experts working on this issue.

Thank you in advance,

Regards,

Subrata Singh

 

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5 Answers

  2. Dear Subrata Singh,

    Firstly, you must use proper terms in your question so as to avoid confusion. Water Harvesting Structure is a generic term used very loosely by everybody loosely connected with water. Terms like ponds, tanks, dams have culture specific connotations.

    I presume you want to prove to someone that your one litre bucket actually holds 100 litre water, as you consume water from it a hundred times. Right? That would help you fetch more price for your bucket.

    If that is so, please read on. You can estimate or measure amount of water your "structure" held and "conserved". As such, you should not use the term "conserved" unless you know what actually happened to that water. You can estimate or measure that for everything from countour trenches on hill slopes to bunds in farmlands to dams and weirs across streams. You need to know the shape and size of your structure - ideally a three dimensional measurement. The geometry will give you the storage capacity. You need to know the rainfall pattern -when it rains and how much. For a rough calculation - use number of rainy days for each month and average rainfall of that month. If in India, use rainy reason months' data depends on your area receives SW or NE or return or a combination of these. This will tell you how much water you should expect per shower/ rainstorm. Use at least 30 years data and calculate 75% dependence, if you want to say something acceptable.

    Then, you need to know your soil AND underground rock type. That will tell you how much water it will be able to HOLD for how much time, and if there is more water, wether it will go underground. In other words, we should know infiltration and percolation component, which depends on soil and underground rock type, structure and its weathering. You should be able to estimate contribution of each rainstorm to soil moisture and groundwater. These, too, are subject to lossess, and one should not boast that all water that goes is a gain for the landowener/ farmer/ village.

    Then, you shoul also know about your sunshine behaviour. That helps you estimate evaporation.

    Once you consider all these aspects, you will get a slight inkling of what is happening to the water you held in your so called water harvesting structure. Then, you can make a presumptive statement about "conserved" water.

    About literature: get any hydrology text book, and you will get the basics. But, before that you must understand the hydrologic cycle well. It is taught in pre-school now, but somewhat scientific explatation is taught in secondary school.

    Please read my note "Introduction to Watershed" to get basic concepts.

    Many empirical studies have been analysed and reported by experts. LVR Reddy and Sebastian Vachiansky estimated recharge benefits of contour bunds, earthfill dams and masonry weirs in Adgaon watershed in 1987. They used about 85 years of daily rainfall data and soil testing at about 50 spots in this 1080 ha watershed. CSWCRTI has few papers.

    Then you may have someone like Kau giving 5% target or parameter. These are very context specific. I can quote few more figures. Mr Vasant Gangawane has suggested to create a storage capacity for surface runoff harvesting of 5%, whereas Mr J M Gandhi gave a figure of 1 Mcft (Million cubic feet) per 125 acres (50 ha) of land. One must understand that while Gangawane worked in zone with 3000 mm rainfall and highly pervious soils, Mr Gandhi worked in area with black soils and 700 mm of annual rainfall. We still use these as guideline values and found them valid even after 30 years. Yet, our design of runoff harvesting is very specific to a location/ site, and we estimate the quantity of recharge for every specific structure.

    We have come across some thumbrules, acceptable to be the metric-driven people, that 3-4 times the storage capacity ("up to the brim") is taken as benefit.

    1 Comment

    1. Thank you a lot, Dr. Yugandhar,

      The information is helpful. Just to clarify, I used the generic term of water harvesting structure because since we work on a watershed concept we have a whole lot of structures ranging from the contours to large water harvesting structures.

      We do have the basic dimensions (3D) of the water harvesting structures to calculate the storage potential. We have been using thumb rules for calculating the multiple refill for the structures based on rainfall pattern - like one refill for more than 20 mm rains for area treatment measures like trenches, bunding, gully plugs etc. and 5 refill a year for rainfall from 500-800 mm rains and 7-9 refills for rainfall above 800 mm.

      We understand that recharge is a factor of infiltration/ percolation rate, type of soil, type of rocks, slope, etc.

      We wanted to know if there are certain established processes for us to calculate the refill. We would be OK with collating information required for the same.

      Seeking your response.

  3. Dear Kau & Eric,

    Thank you for your response. The water harvesting structures are of varied sizes. Some of the water would be used for irrigation in large structures, some structures would be meant for domestic use and for drinking water for livestock. 

    We have been able to calculate the water holding capacity of the structures. I would like to calculate the amount of water that gets recharged during the year because of the structure.

    I am attaching detailed information on the work undertalen in one focation for assistance.

     

  4. Are you looking for capacity calculations? The question is not very clear but as we know,

    capacity of the rain water harvesting storage tank must be the LEAST of either: 5% of the Annual Rainwater Yield (ARY) or 5% of the Annual Rainwater Demand (ARD).

    Answered on by

  5. Your WHS are small ponds or big pond? It will be easy to assist  if you can tell me the capacity of these WHS and also for what purpose the harvested water will be reused?  What is annuma rainfall and slop of the site? 

    2 Comments

    2. Dear Eric,Thank you for your response. The water harvesting structures are of varied sizes. Some of the water would be used for irrigation in large structures, some structures would be meant for domestic use and for drinking water for livestock. Please share your email for me to be able to share the details you have requested for.