2.2 Regional Water Balance
It is instructive to show the opposite hydrological functions of mountain versus flat areas of Central Asia by means of simple water balance considerations.
The water balance equation for mountain area, broadly defined, can be written as follows:
\[\begin{equation} \tag{2.1} p = e + q_{s} + q_{g} \end{equation}\]whereby \(p\) represents the average long-term amount of precipitation and condensation of water vapor from the atmosphere, \(e\) the average long-term evaporation, \(q_{s}\) the average long-term surface outflow and \(q_{g}\) the average outflow of groundwater. This equation shows that the mountain area receives moisture only from the atmosphere and rainfall which precipitates within its limits and evaporates only partially. The remainder part of it flows down in the form of surface and underground drainage. Sharp partition of a relief in the mountain area, and consequently, a deep natural drainage is the reason why groundwater is almost entirely connected to the river network already in the mountain area. The Meso-Cenozoic deposits, containing waterproof horizons, and the Paleozoic massifs on the border with the flat areas obstruct groundwater flows. Thus, the groundwater inflow to the flat areas makes no more than 10% - 15% of the surface one and therefore it can be neglected in the first equation.
Then the water balance equation will have the following appearance:
\[\begin{equation} \tag{2.2} p = e + q_{s} \end{equation}\]Based on available data, the rate of surface outflow \(q_{s}\) can be calculated quite precisely: 155 billion m3 or 201 mm annually. River basin-specific surface runoff values are provided in Table. It is impossible to measure the amount of the accumulated water vapor in the mountains accurately just by observation, so we have to proceed from the rate of the runoff, for which we need to know the value of runoff coefficient. The last can be approximately considered as equal to 0.35 (see also next Section). Then, \(p\) equals 575 mm and \(e\), as follows, 575 mm - 201 mm = 374 mm.
| Basin Name | Area (km2) | Runoff (m3/s) | Runoff entering flatlands (m3/s) | Runoff Coeff. (l/ (s km2)) |
|---|---|---|---|---|
| Caspian Sea | 29’700 | 22 | 12 | 0.74 |
| Endhoreic Basins of TUK and AFG | 193’300 | 180 | 155 | 0.93 |
| Amu Darya | 227’300 | 2’500 | 2’500 | 11 |
| Syr Darya | 150’100 | 1’200 | 1’200 | 8 |
| Chu and Talas River Basin | 37’540 | 190 | 190 | 5.1 |
| Lake Issyk Kul | 12’600 | 115 | - | 9.1 |
| Southern Balkash Lake | 119’000 | 800 | 800 | 6.7 |
| Total | 769’600 | 5’007 | 4’857 | 6.5 |
It should be noted that the rate of surface water inflow to the flatlands is smaller than the runoff which is generated in the mountain areas as part of it is utilized in the mountain area for irrigation purposes (the rivers of Turkmenistan are in this regard an especially good example), or it evaporates from a surface like of the Lake Issyk Kul and other smaller lakes.
E2.1
Exercise on conversions
For water resources specialists it is extremely useful to be able to quickly convert flow number and to put them into context. As an exercise, convert the runoff of one or two Basins in the table above from m3/s to m3/a. Can you derive an approximation for the conversion which you can easily apply when you are in the field and do not have a calculator at hand?
Hint: The number PI is approximately 3.14.
If, from the mountain area, we exclude reservoirs of the river Atrek and the rivers of Turkmenistan and Afghanistan with no runoff which, occupying the big space (29% of all mountain area of Central Asia), excel in a minute quantity of rainfall and exclusively low water levels, separate elements of water balance will be expressed by the following sizes: \(p\) = 675 mm , \(q_{s}\) = 270 mm and \(e\) = 405 mm. In this case, the water balance of mountain area shows its hydrological essence even more clearly.
The equation of water balance for the flat area can be written in the form of
\[ p + q_{i} = e \]
where \(q_{i}\) represents the surface inflow of water.
We neglect underground outflow in the flat area as, even when it takes place, it is absolutely insignificant. The average amount of rainfall calculated by planimetering of the isohyetal map is equal to 173 mm. The rate of inflow of water is equal to the outflow of water from mountain area, i. e. \(q_{s} = q_{i} = 155 \cdot 10^{9} \text{ m} ^{3}\). After making its way down to the flat area, which includes the surface of the Aral Sea and Lake Balkhash, the surface inflow of the rivers reaches 124 mm and the evaporation is \(e = p + q_{i} = 173 \text{ mm} + 124 \text{ mm} = 297 \text{ mm}\). It is noteworthy to mention that from the entire moisture appearing in the flat area, 58% nevertheless is from atmospheric precipitation, despite its rather insignificant absolute amount.
Comparing the two water balance equations shows that the mountain areas receive 575 mm of moisture from the atmosphere of which 374 mm evaporates back to the atmosphere and 201 mm reach the downstream flat area in the form of a surface runoff. Conversely to this, the flat areas receive 297 mm of water from direct precipitation and from inflow of mountain runoff. All of this water evaporates back to the atmosphere.
To summarize, it is clear that in the area of runoff formation, \(p>e\), in the area of runoff losses \(p<e\), and that in the area of runoff balance \(p\approx e\). In each area where the runoff processes show the same orientation, its origin, distribution in time and space, and also the intensity of processes can however vary. In this sense, depending mainly on local topography (generally speaking, depending on the altitude, orientation and exposure of a reservoir to humid air masses), the specific runoff, the persistence of the annual runoff and its distribution over a year, as well as other characteristics of the river flow can sharply differ in different parts of the area of runoff formation, as it was already discussed above. On the other hand, the intensity of runoff losses, their distribution over a year, etc. within the area of runoff losses, considerably depend on the economic activities and features of climatic conditions.