Simulation of River Flow for Downstream Water Allocation in the Heihe River Watershed, Northwest China - Landscape Ecology for Sustainable Environment and Culture - page 176

middle reach oasis reduced discharges at the Zhengyixia Gage Station. thus con-
tributing to the high ratio of modelled to actual mean annual flow volumes of 1.4
achieved in the model calibration for the 1990-2000 model verification period
(Pan and Qian 2001 ; and Wu et al. 2010 ).
Implementation of the State Council's water allocation plan requires taking
about 0.58 9 10 9 m 3 water out of irrigation each year in the middle reach in order
to deliver 0.95 9 10 9 m 3 of water at Zhenyixia Gage Station for rehabilitating the
downstream ecosystem (Pan and Qian 2001 ; The City of Zhangye 2004). This goal
seems achievable during normal climatic conditions. However, governmental
entities (e.g. Cities of Zhangye and Jiuquan) in the middle reach, while coping
with an annual economic loss of about $240 million, must take into account the
uncertainties associated with the simulated mean annual flows while taking a
number of actions such as adjusting crop patterns, water pricing, and market
transfer to deliver more water downstream. Under dry years, a significantly lesser
amount of flow would be available at the Zhengyixia Gage Station, making it much
more difficult to deliver the targeted 0.95 9 10 9 m 3 of water downstream.

9.4 Conclusions

Simulations of the hydrology of the combined upper and middle reaches of the
Heihe River Watershed in Northwest China show that Qilian Mountain in the
upper reach area is the main runoff production area for the entire watershed. On
average, surface runoff and interflow contributed 51 and 49 % of the river flow
respectively for the period of 1990-2000. Annually the river was simulated to
discharge slightly more than 1 9 10 9 m 3 of water from the middle reach (at
Zhengyixia Gage Station) downstream under normal climatic conditions. While
requiring a significant reduction in water withdrawals by water users in the middle
reach, this amount seems to meet the mandate of delivering 0.95 9 10 9 m 3 of
water at Zhengyixia Gage Station for rehabilitation of downstream ecosystems by
the State Council. However, the amount of the flow at the middle reach outlet has
an exceedence probability of 50 % under normal climatic conditions, and is much
less under dry climatic conditions, making it much harder to deliver the required
0.95 9 10 9 m 3 of water downstream. In addition, climate change and rapid urban
expansion will further intensify the water shortage problem in the Heihe River
Watershed. Thus, how to develop a comprehensive water management plan to
address the competing demands for water among agricultural irrigation, industrial
development, urban supplies, and ecosystem protection remains a long term
challenge between water users in the upper, middle, and lower reaches of the
Heihe River Watershed. As limitations in the current structure of the DLBM and
input data led to significant uncertainties in the simulated results, our future work
includes further model refinement and field work to support water resource
decision making in the study watershed and other similar areas of Northwest
China.