River and groundwater interaction

The monthly changes in river TDS, groundwater TDS and water exchange between riverway and groundwater are shown in Fig.4a. River water infiltrates groundwater in high water period and returns to riverway in low water period. The saline regeneration water with the total dissolved solids (TDS) around 3.5 g/l enters riverway in winter, resulting in an increase in TDS level in river water up to 0.8 g/l. In summer, flood flushes the alluvial district with TDS less than 0.3 g/l. The monthly changes in salt exchange between riverway and groundwater are shown in Fig.4b. The annual salt mass move from riverway to groundwater is about 90 thousand tons in summer while the annual salt mass move from groundwater to riverway is about 800 thousand tons in winter. It shows that the river desalinates the alluvial district.

Figure 4: Monthly changes in river TDS, groundwater TDS, water exchange(a) and salt exchange (b).

The water table section A-A and B-B (Fig.2) are observed. The water table level monthly variation averaged from monthly thrice observations from 2000 to 2002 are shown in meter above sea level in Fig. 5. The most western point is about where the Aiximan wetland is and the dashed line shows where the Akesu River is. The observed points are connected using a cubic spline. At the west of the Akesu River in section A-A, the water table level at irrigation area (G5.2 and G5.1) is higher than the water table at Aiximan wetland (G5.2) and also higher than the river in spite of monthly variation of the water table level. At the east of the Akesu River, the irrigation area (G4.4 and G4.3) is also higher than river. These verify that the irrigation area is usual in topographically higher areas. There should be subsurface drainage from irrigation area to the Aiximan wetland and return flow to river. For the section B-B, the water table of irrigation area (G6.1, G6.2, G6.3) is also higher than Aiximan wetland, indicates the subsurface drainage to the wetland. The river and groundwater net exchange direction cannot be determined from the water table sections because the water table at the east of the river (G4.5, G4.8) is lower.

Figure 5: Water table at observed section A-A (a) and B-B (b).