River baseflow of the Cretaceous system in the Ordos Basin, China
|Location||International Geological Congress,oslo 2008|
|Author||Feng, Lihua; Feng, Jiahong|
|Holding Date||17 September 2008|
The Ordos Basin, lying in the east part of Northwest China, is surrounded mostly by mountains, with the total coverage of 270 thousand km2 or so. Whole precipitation in the Ordos Basin is low, while evaporation is strong. However, there are some groundwater resources. Cretaceous system in the Ordos Basin mainly consists of sandstones, whose thickness is between 500-1,100m, which forms a huge good-quality aquifer. According to natural geographic conditions in the Cretaceous system region, and distribution characteristics of watershed, lithology, geological structure, etc, catchments areas with hydrological data in the south region are divided into seven grade areas. On the basis of comprehensive field survey, by using the most advanced AutoCAD software and the technology of baseflow separation, the author suggests a new calculation method of grade area, to make a scientific evaluation on the quantity of the groundwater resources in the Basin.
Baseflow separation is to separate river baseflow amount from river runoff amount at the section of catchment exit. In the separation procedure, as long as the outcome is not reasonable, it needs to be redone. So the calculating workload is huge. Hence, we use the most advanced AutoCAD software to make baseflow separation. because of catchments’ functions of storage and adjustment (pondage action), flood period in the Ordos Basin is after rain season by about half to one month. Hence the four consecutive months with the largest runoff amounts show up from July to October, whose runoff amounts account for 50-70% of the total annual runoff amount. So, when calculating the average annual baseflow over years for each representative station, flood period (July to October) and non-flood period (November to Next June) should be categorized first, and two comprehensive depletion curves should be drawn separately. Then baseflow separations should be conducted on flood period and non-flood period separately.
Considering the average river baseflow modulus over years (M) in each grade area, M of Area I is the largest, which reaches 33.4 thousand m3/km2•a. Ms of Area II and Area VII are the seconds, which are 22.7 and 23.8 thousand m3/km2•a respectively. M of Area V is the smallest, which is only 3 thousand m3/km2•a. Area I is located in the desert area in the upper reaches of Wuding River. Because the percolation into ground is strong in desert area, precipitation will permeate into ground quickly and supply the rivers in the form of groundwater. Hence, the ratio of supplement from groundwater to annual runoff amount is big, which is generally more than 80%.
Therefore, M of Area I is the largest. The calculation shows that river baseflow of Cretaceous system in the Ordos Basin is 1.143 billion m3/a. Among them, baseflow of Wuding River drainage area is 352 million m3/a, which accounts for 30.8%; and baseflow of Jing River drainage area is 538 million m3/a, which accounts for 47.0%.