Simulation of River Flow for Downstream Water Allocation in the Heihe River Watershed, Northwest China - Landscape Ecology for Sustainable Environment and Culture - page 172

grid cell (Croley and He 2005 ; He and Croley 2007a ). Since the soil database of
1999 (1:250,000) from the Gansu Province only contained soil types, we used
SPAW ( So il- P lant- A tmosphere- W ater) Field & Pond Hydrology model (devel-
oped by the U.S. Department of Agriculture Agricultural Research Service and
Natural Resources Conservation Service) to determine relevant soil attributes for
each of the soil types based on the limited soil survey data collected by Xiao and
his group ( 2006 ) Chen and Xiao ( 2003 ). Such attributes include soil texture, depth
of USZ and LSZ, water holding capacity (%) and permeability (cm/hr). Manning's
coefficients were assigned to each cell by the hydrological response units (HRU),
which was determined according to the combination of land use, soil texture, and
slope (He and Croley 2007b ). Average daily river flow rates (in m 3 /s) were con-
verted into daily outflow volumes and used to conduct a systematic search of the
parameter space to minimize the root mean square errors (RMSE) between actual
and simulated daily outflow volumes at the watershed outlet (Croley et al. 2005 ;
Croley and He 2006 ).

9.2.4 Model Calibration and Verification

The DLBRM was calibrated over the period of 1978-1987 for each of the 9,790
cells (4 km 2 ) at daily intervals. The calibration shows a 0.69 correlation between
simulated and observed watershed outflows and a 0.072 mm/d root mean square
error. The ratio of model to actual mean flow was 1.011; and the ratio of model to
actual flow standard deviation was 0.68 (Table 9.3 ). Over a separate verification
period (1990-2000), the model demonstrated a 0.71 correlation between simulated
and observed watershed outflows and a 0.006 cm/d RMSE; the ratio of model to
actual mean flow was 1.409; and the ratio of model to actual flow standard
deviation was 0.72, showing the model overestimated the mean daily flows over
the period of 1990-2000. The simulated annual water budget (averages of the
1990-2000) shows that annual surface net supply from both rainfall and snow melt

Table 9.3 DLBRM Heihe calibration statistics

Long-term average ratio to surface net
supply

Calibration
period

Correlation RMSE

(cm)

l M /
l A

r 2 M = r A Surface

runoff

Interflow GW USZ

ET

LSZ
ET

1978-1987 0.690 0.007 1.014 0.683 0.065 0.061 0.000 0.000 0.884

1978-1987 * 0.690 0.007 1.011 0.682 0.065 0.061 0.000 0.000 0.885

1990-2000 0.710 0.006 1.409 0.717 0.070 0.000 0.000 0.000 0.870

*A 10.85 m snow pack was assumed in about 305 km 2 of mountain area (elevation [4500 m) in

the simulation
ET represents evaporation
l M /l A = ration of simulated mean annual outflow to the actual annual outflow