Simulation of River Flow for Downstream Water Allocation in the Heihe River Watershed, Northwest China - Landscape Ecology for Sustainable Environment and Culture - page 173

was about 8.92 billion (10 9 )m 3 (Fig. 9.3 ), which mainly came from Qilian
Mountain in the upper reach area. The USZ stored about 391 billion m 3 of water,
the largest storage among all the four storage tanks (USZ, LSZ, groundwater zone,
and surface storage). Surface runoff from the USZ averaged about 0.54 billion m 3 ,
while a much larger portion of water (8.37 billion m 3 ) percolated down to the
LSZ. A majority (94 %) of the percolated water evaporated to the atmosphere
from the LSZ and the rest flowed to the stream in the form of interflow. There was
hardly any deep percolation to the groundwater since the LSZ is up to several
hundred meters deep in much of the middle reach area (Pan and Qian 2001 ). The
average annual outflow at the outlet (Zhengyixia) of the middle reach was about
1.05 billion m 3 to the downstream (Fig. 9.3 ) (He et al. 2009 ).
Simulation results of daily flows for 1990 are shown in Fig. 9.4 . Compared to
the observed daily discharge, DLBRM using parameters from both the 1978-1987
and the 1990-2000 calibration periods reasonably depicted the daily variations of
the Heihe discharge at the outlet (near Zhengyixia Station), with the former per-
forming better than the latter. However, the simulations underestimated the dis-
charges during the cold season, when there was not much precipitation, and
overestimated the discharges during the spring and summer, when there were more
storms (except underestimating the discharge from the July 25, 1990 storm of
2.5 cm) (Fig. 9.4 ) (He et al. 2009 ).
9.3 Discussion
The Qilian Mountain (with a peak elevation of 5,500 m above sea level) makes up
the upper reach of the Heihe River Watershed. Due to the high altitude and steep
slope of the mountain area, much of the snow melt and rainfall becomes surface
runoff. Once reaching the mountain outlet (Yingluoxia Gage Station), the water
quickly percolates to the deep, coarse sandy and loamy soils in the alluvial fan (up
to several hundred m deep) which is the main agricultural oasis in the middle reach
(between the mountain outlet at Yingluoxia Gage Station and the middle reach
outlet at Zhengyixia Gage Station) (Cheng et al. 1999 ; Pan and Qian 2001 ; and Wu
et al. 2010 ). As annual precipitation in the oasis is less than 200 mm, the majority
of the river flow is used to irrigate crops like spring wheat, corn and rice in the
oasis, depleting river flow downstream of Zhangye City (Fig. 9.1 ). The simulated
results show that there was hardly any deep percolation to the groundwater.
Instead, a portion of the water in the LSZ was simulated to flow to the river
channels though interflow. This is due to the fact that LSZ is several hundred
meters deep and could be mixed with groundwater zone. Studies by Cheng et al.
( 1999 ), Pan and Qian ( 2001 ), and Wu et al. ( 2010 ) report similar findings that
groundwater recharge is only observable in the middle reach area with ground-
water level less than 5 m deep and daily precipitation more than 10 mm, river flow
infiltrates to the alluvial fan, and then flows out to the river channel from the
aquifer.