Available water capacity is the amount of water that a soil can store that is available for use by plants. It is the water held between field capacity and the wilting point adjusted downward for rock fragments and for salts in solution. Field capacity is the water retained in a freely drained soil about 2 days after thorough wetting. The wilting point is the water content at which sunflower seedlings wilt irreversibly

Dear Libor,

Water availability (WA) is obtained from natural stream flow (QNSF ) by subtracting existing storage (ST), out-of-stream consumptive uses (CU) and in-stream demands (IS).

WA = QNSF – ST – CU – IS

Ideally, water availability would be calculated for every watershed above a point of diversion or instream demand. Practically, the number of watersheds must be limited. The watersheds selected for analysis are called Water Availability Basins (WABs).

With my best regards

Prof. Bachir ACHOUR

Dear Libor

Depending on the type of data available, the water availability can be computed from the following methods:

Direct observation method:

This method is applied when observed runoff data at the proposed site is available for the last 50 years or so.

Rainfall-Runoff series method:

The method consists in extending the runoff data with the help of rainfall data by means of rainfall-runoff relationships .Depending upon the availability of rainfall and runoff data, following three cases arise

• Long term precipitation record along with a stream flow data for a few years is available.

• Long term precipitation record is available for the catchment along with a few years of stream flow data at a neighboring site on the same river.

• Long term precipitation record is available for the catchment rainfall-runoff data on a nearby river.

Langbein’s log-deviation method:

This method is used when short term runoff data is available at the proposed site along with long term runoff at a nearby gauging station.

There are other methods for example:

 Environmental data

Any water resources project would be affecting the environment in one way or other. Construction of a dam or barrage may not allow free movement of fish along the river, the ponded water behind may cause submergence of valuable forest and even human habitation. Construction of flood protection environment may cause water logging in the area behind the embankment unless proper drainage is provided, thus leading to breeding of mosquitoes and other disease carrying vectors.

It is, therefore, always mandatory to check the impact on the environment due to construction of a water resource project. For this purpose, the relevant data on environment and ecology has to be collected for analysis.

 Socio-economic and demographic data

Dam and barrage projects constructed at one point on a river benefits people downstream largely. However, the construction affects the people residing on the upstream as the ponded water causes submergence of villages and force people to migrate. It is pertinent, therefore, to study the effect of the project on the people and impact on the socio-economic fabric of the region benefited or affected by the project.

best wishes

Dear Ammar Kamel, thank you very much for your answer. Can you be more concrete, how compute water availability from run-off data and rainfall data? Thanks

Mr libor ,

Runoff occurs after rainfall satisfies depression storage,potential maximum retention infiltration etc. Antecedent Moisture Condition (AMC)  is an important factor,ie wetness of the soil before the rainfall starts. After satisfying all this runoff starts.

In SCS-CN method of runoff calculation Q= (P-0.02 S)2 /  ( p+0.8S)  P= daily rainfall Q= daily runoff S= retention parameter    all units are in mm    for P>0.2 S

you are concerned with potential maximum retention. ie water available .

Alex