Easy. Compute it as a function of rate over time. The tough part is determining what the infiltration rate is going to be. Any designs based on point measurements will no doubt find very significant heterogeneity in existing soils, which you will disturb during construction anyway, so its findings are of limited utility. The only way to really tell is build it and measure it.

So if you want infiltration to be as low as possible as I assume you do, then you must line the pond with clay. Bentonite is even better but more expensive. BTW, use the SPAW model (google it) to compute ET losses which will still be present.

Dear Deepak

Yes of course! You can estimate this infiltration by using à numerical model or empirical formulas.

For example, you can use numerical models to simulate groundwater flow under the pond (Hydrus does it very well, Modflow ...), and, as surface boundary condition, in contact with the pond, you enter the water level in the pond.

Other boundaries conditions are required in the other limits. They depend on the configuration of your System. In addition, you have to take into account the groundwater table. For example, if it flows into the pond or not,...

Regards

Hocine

Dear Deepak,

you need to know the infiltration rate: it is true (see answer from William Lucas) that point measurements are often variable, but if you measure two/three times around the place where the pond will be it will be an acceptable estimation. Then, with the infiltration rate, you can use some model or, because you want just a season dimensioning, you can directly calculate how much water can be infiltrated in your pond during a season (it will require to know the infiltration area). You can then compare with average monsoon rainfall and with the contributing area.

Regards,

Guido

Assume the position of the pond as saturated zone (pressure=0). Use this as drichilet boundary to condition pressure based Richard's Equation. From the solved pressure field compute the Darcy flux at boundary. It will give the instantaneous inflow which can be integrated (summed) over time to compute the cumulative influx.

Dear Deepak,

I believe that infiltration rate must be directly measured in the area for which a rainwater harvesting structure is supposed to be constructed. In this regard, it is advisable to dig some small pits or ponds and as such final constant rate of infiltration at saturation (ultimate infiltration) should be measured in all the pits and in this case the average of ultimate infiltration rate is calculated for the site and then its rate must be multiplied by the time (here 120 days). So, it will be calculated easily. Of course, even ultimate infiltration rate will be decreased during the time which is resulted form plugging (reducing porosity due to sedimentation on the bottom of the pond) but, it can be ignored if clean water is entered to the pond.

Dear Deepak

Are you planning the rain water harvesting pond for direct use? or for recharging the aquifer?

1) If it is used for recharging the aquifer: You can carry out a simple water balance model. First consider the time step, may be a daily? Estimate the volume of water stored (runoff generated in the catchment area-surplus from your pond in that time step), estimate the evaporation loss (evaporation rate and average water spread area) then the difference will give you the combined loss due to infiltration and bank storage (for all practical purpose the bank storage can be taken as zero), This infiltration loss from the surface water system will become the input to your aquifer water balance.

2) If you are using the rainwater for direct use: Estimate volume of water stored, estimate evaporation loss, estimate the infiltration rate in the water spread area as suggested by William Lucas, and Guido Petrucci. The remaining is the water available for direct use.

Dear Deepak

The infiltration rate is a function of the water depth in the pond, permeability coefficient of the soil, width , length of the pond and side slope of the pond. you may use the published paper in design of canal by Ghazaw to help you.

Thanks Mr. Yousry, Could you share the link of that paper over here for my reference.