In the area of groundwater recharge (Fig. 20.1), a reconstruction of the irrigation system was carried out. As a result, the inflow of groundwater at the expense of irrigational losses has decreased significantly. For this reason, it is necessary to assess changes in the reduction of groundwater flow to subjacent areas of the Chu valley. See a typical cross section in Fig. 20.2.
This problem can be solved by means of ground water modeling. However, a more convenient approach is to derive an approximation formula in which parameters are defined by means of a one-off simulation . This formula can then be used to assess various problems in connection with the effect of groundwater recharge on adjacent hydro-geological areas. A general groundwater flow equation can be written as
Lithologie al symbols
Lithologie al symbols
where Q(t) is the groundwater outflow from the groundwater recharge area into downstream areas of groundwater discharge (assessed value), m3/day; Qs, Qe are the initial and final steady state groundwater outflows (assessed by mass balance), m3/day; e is the base of natural logarithm; t is the period of time, days;
T is the transmissivity (characterizes both groundwater recharge and discharge areas, m2/day; m is the storage coefficient; L is a representative distance from a mountain framework to the border of a discharge area, m; and g(t) is a dimension-less parameter characterizing the specific natural settings of the considered hydro-geological areas, it is assessed by the results of simulation of the area of interest, and it can be time dependent.
The following approach is suggested to estimate g(t) conditions in central and western parts of the Chu valley. In the beginning, the outflow from the area of groundwater recharge within specific periods is estimated based on the simulation results: t1 = 1 year, t2 = 3 year, t3 = 5 years, t4 = 10 years, and t5 = 25 years. By summing up the results, the dependence for g (t) can be computed as follows:
g1 is evaluated by minimization of the function
where Q(ti) is set by expression (20.1) at the time point ti, m3/day; Qi is an outflow of groundwater from a groundwater recharge area into a groundwater discharge area at time ti, m3/day; and g2 is defined by the following dependence:
Estimates using the above equations were done for the western Chu valley. The following parameters were used: T=3,000 m2/day; L = 12,000 m; m=0.2; g and g2 equal to 2.175 and 1.320, respectively. In accordance with one possible development scenario, QS = 3.88 m3/s, QE=0.93 m3/s .
The change of groundwater outflow from the groundwater recharge area in response to reduction of filtration losses is shown in Fig. 20.3. This graph indicates that within about 8 years, the outflow of groundwater into adjacent areas will be reduced to a value of 0.5 (QS - QE). The expected reduction of outflow to a value QS - Qe will occur within 30-40 years. These results suggest that a reduction of groundwater in the area of recharge resulting from climate change will not trigger an immediate change to the hydrologic budget; thereby allowing time for making necessary decisions and system adjustments. Multiple estimates of the response time also can be made with the use of Eqs. 20.1-20.5. As far as the central and western parts of the Chu valley are concerned, the values g1 and g2 can be used in calculations.
Fig. 20.3 Changes of groundwater inflow from the groundwater recharge area into the discharge area at reduction of filtration losses from value QS to value QE
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