when I calculate the runoff volume (in MCM ) at one catchment(total annual runoff cross total area) and calculate the total runoff volume at all considered stations over this catchment(sum of (runoff of each station cross its area), the two volume values suppose to be equal, but whilst I did this calculation I found that the two values are not equal, why was that?
You will probably find that dividing a basin or hydrologic unit into sub parts, watersheds, catchments, etc., you will find parts and pieces of the basin left out. A certain amount of the headwater areas store water underground, and it may only show up downstream or perhaps not at all if it goes to deep groundwater or aquifer storage. Some channels with deep coarse substrates can move water in their hyporheic zone, and not be measured. Take for example ephemeral stream catchments, most of the rainfall is absorbed and seldom any runoff. The absorbed water in these areas may be used in evapotranspiration, or move slowly downstream and show up eventually in groundwater or downstream. Some streams are gaining, that is they gain flow from adjacent water table, and some are loosing streams as some of the water in the stream is lost to the adjacent groundwater. In addition, geologic faults or shear lineaments or karst geology may remove water as it flows downstream, such as is the case of the Suwannee River in Georgia and Florida. And sometimes karst geology includes springs that appear and can move water from underground sources outside of the hydrologic surface divides. Rainfall distribution may be unequal, producing runoff in one area, but absorbed in downstream substrates.
Surface area such as a roof or yard, multiply the area by the inches of rainfall and divide by 231 to obtain the runoff in gallons. The factor 231 comes from the fact that the volume of 1 gallon equals 231 cubic inches.
Dear Alaa,
It would be better, if you can elaborate your question then only I will be able to reply to your question in proper way.
You will probably find that dividing a basin or hydrologic unit into sub parts, watersheds, catchments, etc., you will find parts and pieces of the basin left out. A certain amount of the headwater areas store water underground, and it may only show up downstream or perhaps not at all if it goes to deep groundwater or aquifer storage. Some channels with deep coarse substrates can move water in their hyporheic zone, and not be measured. Take for example ephemeral stream catchments, most of the rainfall is absorbed and seldom any runoff. The absorbed water in these areas may be used in evapotranspiration, or move slowly downstream and show up eventually in groundwater or downstream. Some streams are gaining, that is they gain flow from adjacent water table, and some are loosing streams as some of the water in the stream is lost to the adjacent groundwater. In addition, geologic faults or shear lineaments or karst geology may remove water as it flows downstream, such as is the case of the Suwannee River in Georgia and Florida. And sometimes karst geology includes springs that appear and can move water from underground sources outside of the hydrologic surface divides. Rainfall distribution may be unequal, producing runoff in one area, but absorbed in downstream substrates.
In addition, stream gauging is seldom exact, and if you are dealing with small catchments that have rapid or flashy response, you may need to measure water level in flume or weir every 5 minutes to be sure you are characterizing the hydrographs sufficiently. When using flumes or weirs, the guidance is also to use a concrete barrier to bedrock or impermeable substrate. If using standard wading flow meter such as Type AA or pigmy meter, a good measurement is probably within the range of plus or minus 10-20%. Even rated and carefully constructed flumes can have some temporary issues with leaves or woody debris, sediment, etc. Low gradient systems with connecting or adjacent wetlands, beaver dams, etc. might influence. And if there are any diversions of water for irrigation or other uses, this would complicate your analysis.
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