1. Introduction
Flood disasters, characterized by their frequent occurrence, have always been one of the most severe natural disasters in China. According to incomplete statistics, it is accounted that more than two-thirds of the total flood-related fatalities are attributed to floods in small and medium-sized rivers [
1]. Economic losses in China, primarily induced by floods in small and medium rivers, have increased a lot in recent years by the rise in extreme weather events. Frequent extreme precipitation in these river basins has led to severe flood disasters [
2]. Therefore, Monitoring, forecasting, and early warning of flood disasters in small and medium rivers have received significant attention from the government and society.
Extensive mechanistic research and model development for flood prediction and early warning in small and medium rivers have been conducted domestically and internationally. Prevailing research on the mechanisms behind the formation of small to medium-sized rivers predominantly focuses on precipitation, soil moisture, soil types, vegetation traits, and topographical features [
3,
4,
5,
6,
7]. With the development of hydrological theories, efforts have been made globally to develop hydrological models, and a series of hydrological models such as the Liuxihe model, GBHM, Xin’anjiang model, HEC-HMS, HBV, SHE, TOPMODE, and parameter optimization methods have been proposed [
8,
9,
10,
11,
12,
13,
14,
15], among which distributed and semi-distributed hydrological models are the main research directions. In recent years, owing to the maturation and enhancement of ArcGIS technology, distributed hydrological models have progressively become the predominant research focus [
16]. The HEC-HMS distributed hydrological model is one of the HEC series hydrological models developed by the Hydrologic Engineering Center of the US Army Corps of Engineers, primarily used for flood simulation in basins. The HEC-HMS model integrates mainstream runoff calculation methods and allows for the flexible combination of multiple modules such as runoff, production, concentration, and base flow. It can simulate and predict runoff processes under different conditions effectively [
17,
18,
19]. Therefore, the HEC-HMS model has been widely used in flood prediction and early warning research in small and medium river basins both domestically and internationally [
20,
21,
22,
23,
24,
25].
Zema et al. [
26]applied the HEC-HMS model in small basins in semi-arid regions to assess the impact of parameter accuracy on results. Sharafati et al. [
27] studied the uncertainty of rainfall and HEC-HMS hydrological parameters on flood prediction and found that flood volume is more sensitive to parameter uncertainty. Wijayarathne et al. [
28] compared five distributed and semi-distributed hydrological models and found that the HEC-HMS performed well in 1-3 days of runoff simulations, recommending it for actual flood forecasting. Yuan Wenlin [
29] compared the critical rainfall determined by HEC-HMS and instantaneous unit hydrograph (IUH) models and found that HEC-HMS has higher accuracy in short-duration warnings. Zhang Jianjun [
30], Cheng Xu [
31], and Ren Juanhui [
32] explored the applicability of the HEC-HMS model in small and medium river basins in China, finding that the HEC-HMS model could better simulate the entire flood process, providing a basis for hydrological forecasting in small and medium river basins without data in China. Zhang Shanshan [
33], Zhang Bo [
34], and Ma Tianhang [
35] simulated the flood events in small and medium river basins in China based on the HEC-HMS model, using the calibrated and validated model to determine the critical rainfall for the corresponding basins. Their research results can be used for flood disaster forecasting and early warning services.
The southern part of Gansu Province, where the Loess Plateau and the Qinghai-Tibet Plateau intersect, is characterized by the highest frequency of flood disasters in small and medium rivers within the province, suffering significant economic losses, high mortality rates, and extensive territorial impact [
36]. In recent years, research on flood disasters in small and medium river basins in Gansu has mainly focused on the physical mechanisms of flood occurrence and meteorological risk early warning [
37,
38]. However, research on flood disaster prediction and early warning combined with hydrological models is relatively scarce, mainly due to the complex geographical factors, difficulty in obtaining basic data, and incomplete research methods in Gansu. With the development and application of hydrological models in flood prediction and early warning, developing proper flood models and early warning indicators suitable for Gansu is a new direction. Consequently, the mountainous XiHanShui River Basin in southern Gansu has been selected as the focal point of this study, which involves constructing an HEC-HMS flood model. This model is then calibrated and validated using observed runoff data to explore its suitability for application within the basin. Based on the calibrated HEC-HMS model, the critical areal rainfalls were determined by using the model testing method, providing technical support for flood disaster prediction and early warning in the XiHanShui River Basin.
6. Conclusion and Discussion
6.1. Conclusion
This study focused on the Xihan River Basin as the research area, employing flood events data from May to October between 2020 and 2021 to calibrate and verify the parameters of the HEC-HMS model. The purpose of this approach is to evaluate the performance of the proposed model at the XiHanShui River Basin and to utilize the model in determining the critical areal rainfall for flood disasters within the basin.The following main conclusions were drawn:
The HEC-HMS model parameters for the XiHanShui River Basin were set as follows: initial loss of 2.0 mm, direct runoff storage coefficient of 0.2 hours, base flow index decay constant of 0.03 hours, channel routing lag time of 0.32 hours, and channel storage coefficient of 0.72 hours. The stable infiltration rate was concentrated between 0.06 mm/h and 0.10 mm/h, with a median of 0.08 mm/h. The runoff concentration time was concentrated between 22.5 hours and 24.1 hours, with a median of 24.0 hours. The impermeable area ratio was concentrated between 4.9% and 5.5%, with a median of 5.1%. Additionally, the impermeable area ratio, stable infiltration rate, and runoff concentration time all followed an unimodal distribution.
The HEC-HMS model was used to simulate the flood events. During the calibration period, REp and REV were within 20%, ∆T was within 1 hour, and NSE was above 0.73. During the validation period, REp and REV were within 20%, ∆T was within 1 hour, and NSE was above 0.74. The average REp was 7.56%, the average REV was 14.76%, the average ∆T was -0.5 hours, and the NSE was 0.769, indicating that the HEC-HMS model has an ideal flood simulation effect for the XiHanShui River Basin.
Using the calibrated HEC-HMS model, the critical areal rainfalls were determined through the model testing method. The 24-hour warning areal rainfall and guaranteed areal rainfall for Pingluo hydrological station were 108.1 mm and 230.8 mm, while for Tanjiaba hydrological station, they were 128.5 mm and 184.6 mm. Based on the warning areal rainfall and guaranteed areal rainfall for Pingluo and Tanjiaba hydrological stations, reasonable early warnings can be provided for the flood events in the XiHanShui River Basin.
6.2. Discussion
Results of this study confirmed the high reliability of the HEC-HMS model for flood simulation in the XiHanShui River Basin. The manual trial-and-error method and PRMSE as the objective function can effectively improve simulation accuracy, although some aspects need further optimization, especially in reducing the relative error of runoff volume. Therefore, future work on flood simulation based on the HEC-HMS model should focus on three aspects:
Optimize the collection and processing of data: Use hydrological data with shorter time steps to better reflect flow process lines. Employ more accurate precipitation measurement techniques, such as radar or satellite-derived precipitation data combined with surface station data, to reduce spatial and temporal errors.
Optimize the Structure and Parameter of Model: Refine the soil moisture model, including dynamic changes in soil moisture content, infiltration rate, and the impact of vegetation changes on soil moisture. Calibrate the model using historical flood event data and validate it with data from different scenarios to improve model accuracy and robustness.
Comprehensive Consideration of Error Sources: Consideration must be given to errors in data and model structure, as well as other potential sources, including measurement inaccuracies and parameter uncertainties.Then attempt to quantify the impact of these errors on model outputs.