Abstract
Abstract. To solve the problem of estimating and verifying stream flow without direct observation data, we estimated stream flow in ungauged zones by coupling a hydrological model with a hydrodynamic model, using the Poyang Lake basin as a test case. To simulate the stream flow of the ungauged zone, we built a soil and water assessment tool (SWAT) model for the entire catchment area covering the upstream gauged area and ungauged zone, and then calibrated the SWAT model using the data in the gauged area. To verify the results, we built two hydrodynamic scenarios (the original and adjusted scenarios) for Poyang Lake using the Delft3D model. In the original scenario, the upstream boundary condition is the observed stream flow from the upstream gauged area, while, in the adjusted scenario, it is the sum of the observed stream flow from the gauged area and the simulated stream flow from the ungauged zone. The experimental results showed that there is a stronger correlation and lower bias (R2 = 0.81, PBIAS = 10.00 %) between the observed and simulated stream flow in the adjusted scenario compared to that (R2 = 0.77, PBIAS = 20.10 %) in the original scenario, suggesting the simulated stream flow of the ungauged zone is reasonable. Using this method, we estimated the stream flow of the Poyang Lake ungauged zone as 16.4 ± 6.2 billion m3 a−1, representing ∼ 11.24 % of the annual total water yield of the entire watershed. Of the annual water yield, 70 % (11.48 billion m3 a−1) is concentrated in the wet season, while 30 % (4.92 billion m3 a−1) comes from the dry season. The ungauged stream flow significantly improves the water balance with the closing error decreased by 13.48 billion m3 a−1 (10.10 % of the total annual water resource) from 30.20 ± 9.1 billion m3 a−1 (20.10 % of the total annual water resource) to 16.72 ± 8.53 billion m3 a−1 (10.00 % of the total annual water resource). The method can be extended to other lake, river, or ocean basins where observation data is unavailable.
Highlights
In recent years, floods and droughts have occurred frequently (Cai et al, 2015; Tanoue et al, 2016), threatening lives and health, reducing crop yields, and hindering economic development (Lesk et al, 2016; Smith et al, 2014)
A method coupling hydrology and hydrodynamics can be used to simulate and verify stream flow in ungauged zones, solving the simulation and verification problems caused by the unavailability of stream flow observations
We estimated that the ungauged zone of Poyang Lake produces a stream flow of approximately 16.4 billion m3, representing approximately 11.4 % of the total inflow from the entire watershed
Summary
Floods and droughts have occurred frequently (Cai et al, 2015; Tanoue et al, 2016), threatening lives and health, reducing crop yields, and hindering economic development (Lesk et al, 2016; Smith et al, 2014). To reduce the damage to the population, agriculture, and economy, we should attempt to predict floods and droughts precisely. In watersheds, ungauged zones lack stream flow observations. The ungauged stream flow is difficult to estimate and is usually neglected in water yield estimations, which can result in flood/drought predictions being not accurate enough. These ungauged zones are an area of interest in ungauged basins (Sivapalan et al, 2003). Ungauged zones, which stretch from the downstream boundary of a gauged basin to the upper boundary of an adjacent water body, exist in river, lake, and ocean catchments universally.
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