Simulation of River Flow for Downstream Water Allocation in the Heihe River Watershed, Northwest China - Landscape Ecology for Sustainable Environment and Culture - page 169

Station). Competitions for the limited water resources have caused intense conflicts
between water users in the Hexi Corridor and those in Alashan Highland in the lower
reach. To address this pressing issue, the State Council has mandated the allocation
of 0.95 9 10 9 m 3 of water to the lower reach under normal climatic conditions for
rehabilitating the downstream ecosystems. Implementation of such a mandate
means reducing the current water use by 0.58 9 10 9 m 3 for agricultural irrigation
(potentially taking about 40,000 ha of farmland out of irrigation) and domestic
supply in Zhangye City alone in the middle reach (Pan and Qian 2001 ), a potential
loss of about $240 million annually for the city.
9.2.2 Description of the DLBRM
To answer the question, ''How much water flows downstream from the upper and
middle reaches (at the Zhengyixia Gage Station) in the Heihe Watershed?'', this
study uses the Distributed Large Basin Runoff Model to simulate the hydrology of
the combined upper and middle reaches of Heihe River Watershed at daily
intervals over the period of 1978-2000. The DLBRM was developed by the U.S.
National Oceanic and Atmospheric Administration (NOAA)'s Great Lakes Envi-
ronmental Research Laboratory and Western Michigan University. It represents a
watershed by using 1 km 2 (or other size) grid cells. Each cell of the watershed is
composed of moisture storages of the upper soil zone (USZ), lower soil zone
(LSZ), groundwater zone (GZ), and surface, which are arranged as a serial and
parallel cascade of ''tanks'' to coincide with the perceived basin storage structure
(Fig. 9.2 ). Water enters the snow pack, which supplies the basin surface (degree-
day snowmelt). Infiltration is proportional to this supply and to saturation of the
upper soil zone (partial-area infiltration). Excess supply is surface runoff. Flows
from all tanks are proportional to their amounts (linear-reservoir flows). Mass
conservation applies for the snow pack and tanks; energy conservation applies to
evapotranspiration (ET). The model computes potential ET from a heat balance,
indexed by daily air temperature, and calculates actual ET as proportional to both
the potential and storage. It allows surface and subsurface flows to interact both
with each other and with adjacent-cell surface and subsurface storages. The model
has been applied extensively to riverine watersheds draining into the North
America's Laurentian Great Lakes for use in both simulation and forecasting
(Croley and He 2005 , 2006 ; He and Croley 2007a , 2010 ; DeMachi et al. 2011 ;
Croley et al. 2005 ). The unique features of the DLBRM include: (1) use of readily
available climatological, topographical, hydrological, soil, and land use databases;
(2) applicability to large watersheds; and (3) analytical solutions for mass conti-
nuity equations, (mathematical equations are not shown here due to space limi-
tations; for details, see Croley and He 2005 , 2006 ; He and Croley 2007a ).
The DLBRM requires 16 input variables for each of the grid cells. To facilitate
the input and output processing for the DLBRM, an ArcView-DLBRM
(AVDLBRM) interface program has been developed to assist with the model