You will need to carry out a water balance study of the reservoir to be able to do this. However, you will first need to determine the reservoir surface area and the storage using the level data and the storage-area-storage relationship at the reservoir site. I'm sure that the reservoir site you are looking at has such a function. In addition, you will also need the time series of the releases from the reservoir and the time series of the open water evaporation at the site. With these available, all you need to do to get the information you require is as follows:

1. Use the storage-area-height function to convert the time series of the heights into time series of reservoir storage (s) and reservoir surface area (A)

2. Use the surface area to convert the evaporation depth into evaporation volume.

3. Apply the reservoir mass balance equation sequentially to the data to get the inflows as follows:

I(t) = s(t+1)-s(t) + o(t) + e(t)

Where I(t) is the required inflow in period t, s(t+1) is the storage at the end of t, s(t) is the storage at the beginning of t, o(t) is the total release (including any spills and compensation) during period t, and e(t) is the volumetric evaporation during period t.

I trust that the above is helpful but if you want further information, you should contact McMahon, TA & Adeloye, AJ (2005): Water Resources Yield. Water Resources Publications, Colorado, USA (wwww.wrpllc.com).

To add to the previous answer: if you don't have the level-storage relationship, you can build one if you have a topographic map. And if you don't have evaporation, you can estimate it from potential evapotranspiration calculated from meteorology (see the SWAT theory manual for a method).

Yes but what you need is the open water surface evaporation; not the evapotranspiration. If you obtain the reference evapotranspiration using SWAT's approach, remember that you will need a small (upward) adjustment to obtain the evaporation. And finally, don't forget to also account for the direct rainfall on the reservoir surface. All of this is fully described by McMahon & Adeloye (2005) that I referred you to in my earlier contribution.

Adebayo's explanation is good. However, you might wish to consider two more issues.

Depending on hydrogeology of your site, or more precisely, of reservoir bottom, there might be losses of water through seepage. By the way, SWAT does take it into account.

Secondly, inflow caluclated using the method calculated by Adebayo will be total inflow to reservoir and I'm not sure if you're interested in total inflow or in inflow that comes through the main river (that is not necessarily the same). I guess there is also surface and sub-surface runoff that reaches reservoir directly (is not channelised). There also might be small tributaries having outlets in reservoir. So in principle, much depends on the difference between drainage areas at the reservoir outlet and inlet from the main river.

Assuming that the dam is a run of the river dam (not water supply or irrigation etc). Start by rating or obtain the rating curve for the spillway depending on the shape of the spillway various forms of the weir equation may be adequate. That will give you outflow and you can back your way into an inflow estimate using the other answers.

I am so overwhelmed with joy by contributions from Adebayo Adeloye, J.P. Nunes and Mikołaj Piniewski to my question. You have made my day. Your contributions are now the key to the door of my research work. I am so very, very grateful and particularly to Adebayo Adeloye. I pray and thank God for the lives of selfless and great minds like you people. I feel, as well very privilege to be on Research gate.

1) To Adebayo Adeloye: The reservoir site I am working on has the storage-area-storage relationship function, thus your proposed approach is great and will be so helpful.

2) To J.P. Nunes: The site has a topographic map and evaporation data as well. Thanks so much for that information.

3) To Mikołaj Piniewski: I am actually much interested in total inflow to the reservoir - the reason I am much happy about the approached given by Adebayo Adeloye. Thanks so much for your reminder. I do greatly appreciate it.

Once again, I am so overwhelmed and so very grateful to you all for your kind contributions. God richly bless you all.

Thank you so much, Gregory Perry, for your kind contribution. I appreciate that so much.

The dam is a dual purpose one, serving as water supply and irrigation. However, you contribution still gives me a helpful picture on what to do. Thanks so much.

Yes but what you need is the open water surface evaporation; not the evapotranspiration. If you obtain the reference evapotranspiration using SWAT's approach, remember that you will need a small (upward) adjustment to obtain the evaporation. And finally, don't forget to also account for the direct rainfall on the reservoir surface. All of this is fully described by McMahon & Adeloye (2005) that I referred you to in my earlier contribution.

I have understood and noted very well all the wonderful points made by everyone of you in your contributions. These are of great help to me. Thanks so very much to you all and may GOD richly bless all of you.

Thank you so much, Philippe Vauchel, for your idea. I am so grateful.