Rainfall and land use are intricately interconnected, influencing each other in several significant ways. The changes that occur in land use, such as urbanization or deforestation, can alter the natural water cycle, impacting both the intensity and rainfall distribution. Conversely, rainfall patterns influence land use decisions, especially in agriculture, where the amount and timing of precipitation often dictate the choice of crops and farming practices. This dynamic relationship highlights the importance of considering both environmental planning and resource management factors.
3.1. Simulation of River Flooding under Different Rainfall Characteristics Scenarios
Rainfall return periods of 5, 10, and 50 years with uniform spatial distribution of rainfall. The flood evolution process under different return periods with a uniform spatial rainfall distribution was simulated. The hydrograph, peak flow, and peak water depth at the outlet section of the basin under different return periods with a uniform spatial distribution of rainfall are shown in
Figure 4 and
Table 5.
Figure 4 shows that the flow at the outlet section of the basin rapidly increases with time, reaching its peak between 5400 s and 7200 s, and then slowly decreases to a stable level. Both scenario R1 and scenario R2 exhibit a flow value close to the peak flow, which can occur either before or after the peak flow. Scenario R3, on the other hand, does not show a flow value close to the peak flow. Instead, it experiences a sudden drop after the peak flow, followed by a subsequent rise and a slow decline to stability. The peak flow in scenario R1 is 1332.90 m
3/s, occurring at 7200 s, with a peak water depth of 5.42 m at 9000 s. In scenario R2, the peak flow is 1445.33 m
3/s, occurring at 5400 s, with a peak water depth of 5.44 m at 9000 s. Scenario R3 has a peak flow of 2003.04 m
3/s, occurring at 5400 s, with a peak water depth of 8.57 m at 9000 s. It can be observed that both the peak flow and peak water depth increase with the increase in the return period. However, when the return periods are relatively close, such as 5 years and 10 years, the increase in peak flow and peak water depth is not significant. Furthermore, as the return period increases, the timing of the peak flow advances. There will be changes in future precipitation due to global warming and the intensification of the water cycle process [
63,
64].
The spatial distribution of rainfall is uneven, with a return period of 5, 10, and 50 years. Simulate the flood evolution process under different return periods of uneven rainfall. The flow hydrograph, peak flow, and peak water depth of the watershed outlet section under different return periods of uneven rainfall are shown in
Figure 5 and
Table 6. Scenario R4 and Scenario R8 have significant differences in the discharge hydrograph of the watershed outlet section at different return periods, while Scenario R5, Scenario R6, and Scenario R7 have little difference in the discharge hydrograph of the outlet section at different return periods. The peak flow rates of scenarios R4, R5, R6, and R7 with different return periods all appeared at 5400 s and then gradually decreased with time. The peak flow of scenario R8 with different return periods occurs between 5400 s and 7200 s and then slowly decreases to a stable state. The outlet section discharge hydrograph of scenario R6 and scenario R7 is not very different; that is, when the rainfall center is in the upstream and midstream, the impact on the basin flood is relatively close. Scenario R8 is quite different from scenarios R6 and R7, which indicates that when the rainfall center is downstream, the impact on the basin flood is different from that in the middle and upstream.
Scenario R4 has a peak flow rate of 1836.19 m3/s, 1954.21 m3/s, and 2277.76 m3/s during the 5, 10, and 50-year return periods, respectively, which is 37.76%, 35.21%, and 13.72% higher than the uniform distribution of rainfall during the same return period. The peak water depths are 5.38 m, 5.41 m, and 7.39 m, respectively, 0.74%, 0.55%, and 13.77% less than the uniform rainfall distribution. Scenario R5 has a peak flow rate of 1825.59 m3/s, 1894.72 m3/s, and 1949.12 m3/s under three different return periods, respectively. Compared with the same return period, the rainfall distribution uniformly increased by 36.96%, 31.09%, and decreased by 2.69%. The peak water depths were 5.61 m, 5.39 m, and 5.40 m, respectively, which increased by 3.51%, decreased by 0.92%, and 36.99% compared to the uniform distribution of rainfall. Scenario R6 has a peak flow rate of 1813.82 m3/s, 1883.36 m3/s, and 2023.07 m3/s during the 5, 10, and 50-year return periods, respectively, which is 36.08%, 30.31%, and 1.00% higher than the uniform distribution of rainfall during the same return period; The peak water depths were 5.60 m, 5.39 m, and 5.86 m, respectively, which increased by 3.32%, decreased by 0.92%, and 31.62% compared to the uniform distribution of rainfall.
Scenario R7 has a peak flow rate of 1813.73 m3/s, 1883.31 m3/s, and 2023.22 m3/s during the 5, 10, and 50-year return periods, respectively, which is 36.07%, 30.30%, and 1.01% higher than the uniform distribution of rainfall during the same return period. The peak water depths were 5.60 m, 5.39 m, and 5.86 m, respectively, which increased by 3.32%, decreased by 0.92%, and 31.62% compared to the uniform distribution of rainfall. Scenario R8 has a peak flow rate of 1883.34 m3/s, 2023.48 m3/s, and 2694.34 m3/s during the 5, 10, and 50-year return periods, respectively, which is 41.30%, 40.00%, and 34.51% higher than the uniform distribution of rainfall during the same return period. The peak water depths were 5.39 m, 5.71 m, and 8.82 m, respectively, which decreased by 0.55%, increased by 4.96%, and 2.92% compared to the uniform distribution of rainfall.
Figure 5 shows that the peak flow values and peak water depths for the 5-year and 10-year return periods are relatively close across different scenarios, while there is a significant difference between the 10-year and 50-year return periods. It is quite different from the peak water depth. It is further explained that when the difference between return periods is larger, the difference between peak flow rate and peak water depth is larger. The peak flow and peak water depth under the above four scenarios with uneven rainfall spatial distribution are quite different from those under uniform rainfall spatial distribution. The largest difference is scenario R8, and the one closest to the uniform rainfall scenario is scenario R7. It can be seen that when the return period is small, the change rate of peak flow is larger, and when the return period is larger, the change rate of peak flow is smaller. It can be seen that when the return period reaches a certain level, such as the 100-year return period, the peak discharge at the outlet section of the basin is relatively close under the scenarios of uneven rainfall distribution and uniform rainfall distribution.
The simulated peak flows of Scenario R4, Scenario R5, Scenario R6, and Scenario R7 under the 5-year return period are relatively close, indicating that when the return period is small, the heavy rain center is in the southeast, southeast, and northwest, the upper reaches of the basin and the middle reaches of the basin. The degree of flood impact in the Hulu River Basin is relatively similar. The peak flows simulated by scenario R8 are significantly different from those of other scenarios in all return periods, indicating that when the center of heavy rain is in the lower reaches of the basin, the impact on floods in the basin is stronger. At this time, special attention should be paid to the safety of the rivers downstream of the basin, and disaster prevention and reduction measures should be taken.
3.2. Simulation of River Flooding under Different Land Use Scenarios
The historical inversion method is used to simulate the four historical land use scenarios, and the changes in the simulation results between two adjacent periods are considered as the degree of land use impact on floods. The peak flow and peak water depth for different return periods under each scenario are shown in
Table 7 and
Table 8, respectively. The flood hydrographs at the watershed outlet for different land use scenarios are shown in
Figure 6.
Table 7 shows that compared to Scenario L1, Scenario L2 shows an increase in peak flow of 20.26%, 14.68%, and 17.14% for the 5, 10, and 50-year return periods, respectively. The peak water depth increases by 0.37%, 9.07%, and 3.80%, respectively. Compared to Scenario L2, Scenario L3 shows an increase in peak flow of 4.92%, 4.72%, and 1.86%, and an increase in peak water depth of 1.66%, 6.62%, and 2.63% for the 5, 10, and 50-year return periods, respectively. Compared to Scenario L3, Scenario L4 shows a decrease in peak flow of 25.75%, 26.18%, and 21.39%, and a decrease in peak water depth of 1.45%, 13.38%, and 4.57% for the 5, 10, and 50-year return periods, respectively. It can be observed that there is a significant variation in peak flow between two adjacent land use scenarios, while the variation in peak water depth is less than 15%. This indicates that the changes in land use between two adjacent periods have a greater impact on peak flow and a relatively smaller impact on peak water depth.
In Scenario L1 compared to Scenario L4, the peak flow increased by 6.74%, 12.80%, and 6.62% for different return periods, while the peak water depth decreased by 0.55%, 0.74%, and 1.63% for the 5, 10, and 50-year return periods, respectively. In Scenario L2 compared to Scenario L4, the peak flow increased by 28.37%, 29.36%, and 24.89% for different return periods, and the peak water depth decreased by 0.18% for the 5-year return period but increased by 8.27% and 2.10% for the 10 and 50-year return periods, respectively. In Scenario L3 compared to Scenario L4, the peak flow increased by 34.68%, 35.47%, and 27.22% for the 5, 10, and 50-year return periods, respectively, and the peak water depth increased by 1.48%, 15.44%, and 4.78% for the corresponding return periods. Thus, it can be observed that during the period from 1985 to 2020 [
65,
66], the changes in land use resulted in an increasing trend followed by a decreasing trend in both peak flow and peak water depth in the watershed.
The flood evolution process was simulated and analyzed for three land use scenarios, including two integrated land use scenarios and one scenario considering generalized water conservation measures, compared to the current land use scenario (Scenario L4). The simulation results and the changes compared to the current land use scenario are shown in
Table 8. From the table, it can be seen that the peak flow differs significantly among the two integrated land use scenarios and the scenario with generalized water conservation measures for different return periods, while the variation in peak water depth is relatively small. Compared to Scenario L4, the peak flow decreased by 57.34%, 45.92%, and 32.63% for the 5-year return period in Scenarios L5, L6, and L7, respectively. Similarly, for the 10-year and 50-year return periods, the peak flow decreased by 59.19%, 47.73%, 28.33%, and 59.91%, 54.46%, and 28.79%, respectively. The corresponding reductions in peak water depth were 11.62%, 9.04%, and 2.95% for the 5-year return period, 11.76%, 8.64%, and 2.57% for the 10-year return period, and 41.89%, 40.49%, and 35.47% for the 50-year return period.
Furthermore, according to
Figure 6, the peak flow in Scenarios L5, L6, and L7 occurs later than in Scenario L4, approximately between 3600 s and 7200 s. This indicates that not only do the peak flow and peak water depth decrease significantly for different return periods, but they also occur later. It suggests that the measures of returning farmland to forest and grassland have a pronounced inhibitory effect on both peak flow and peak water depth. The water conservation measures also contribute to a delay in peak flow and peak water depth. This is because the implementation of water conservation measures retains most of the runoff locally, leading to a decreasing trend in peak flow and peak water depth at the outlet section of the watershed.
The main land use types in the Hulu River Basin are cultivated land and grassland, supplemented by woodland, shrubs, water bodies, unused land, and impermeable areas. Cultivated land has the highest proportion in all four periods, exceeding 60% and being the largest land use type. Grassland has the second largest proportion, also exceeding 20%. In these four periods, the proportion of arable land shows a trend of first increasing and then decreasing, accounting for 60.09%, 75.62%, 72.94%, and 62.66% of the total area, respectively. The proportion of grassland shows a trend of decreasing first and then increasing, accounting for 38.02%, 22.30%, 24.78%, and 34.10%, respectively. The proportion of woodland gradually increases, accounting for 1.71%, 1.89%, 2.04%, and 2.81%, respectively. The proportion of shrubs shows a slight increase, accounting for 0.009%, 0.006%, 0.005%, and 0.017% of the total area. The proportion of water bodies and unused land initially decreases and then increases, while the proportion of impermeable areas gradually increases. Between 1985 and 2020, except for a 3.92% decrease in grassland area, the area of cultivated land, woodland, shrubs, water bodies, unused land, and impermeable areas all increased. The increase in proportion is 2.57%, 1.10%, 0.01%, 0.05%, 0.01%, and 0.18%, respectively. In short, the Hulu River Basin has been mainly characterized by cultivated land and grassland, with increasing proportions of woodland and shrubs. Water bodies, unused land, and impermeable areas have also shown changes in their proportions (
Table 9).
3.3. Simulating the Interacting Effects of Rainfall and Land Use Characteristics on River Flooding
By taking the peak flow and peak water depth obtained from the simulation of the 50-year return period uniform rainfall and land use scenario as the baseline and comparing and analyzing the simulation results under different comprehensive scenarios, we can determine the degree of impact of the combined changes in rainfall characteristics and land use on floods. The simulation results for scenarios RL1-1 to RL1-6 are shown in
Table 10, and the outflow hydrographs are shown in
Figure 7a. The peak flow occurs at 5400 s for all scenarios. In scenario RL1-1, the peak flow is 2313.54 m
3/s, and the peak water depth is 7.47 m, representing a 15.50% increase in peak flow and a 12.84% decrease in peak water depth compared to the baseline period. In scenario RL1-2, the peak flow is 2377.32 m
3/s, representing an 18.69% increase, and the peak water depth is 8.17 m, representing a 4.67% decrease. In scenario RL1-3, the peak flow is 2424.66 m
3/s, and the peak water depth is 8.36 m, representing a 21.05% increase in peak flow and a 2.45% decrease in peak water depth compared to the baseline period. In scenario RL1-4, the peak flow is 1902.56 m
3/s, and the peak water depth is 5.39 m, representing a 5.02% decrease in peak flow and a 37.11% decrease in peak water depth compared to the baseline period. In scenario RL1-5, the peak flow is 2002.31 m
3/s, and the peak water depth is 5.42 m, representing a 0.04% decrease in peak flow and a 36.76% decrease in peak water depth compared to the baseline period. In scenario RL1-6, the peak flow is 2277.76 m
3/s, and the peak water depth is 7.39 m, representing a 13.72% increase in peak flow and a 13.77% decrease in peak water depth compared to the baseline period.
The simulation results for scenarios RL2-1 to RL2-6 are shown in
Table 11, and the outflow hydrographs are shown in
Figure 7b. The peak flow occurs at 5400 s for all scenarios. In scenario RL2-1, the peak flow is 1989.97 m
3/s, and the peak water depth is 5.42 m, representing a 0.65% decrease in peak flow and a 36.76% decrease in peak water depth compared to the baseline period. In scenario RL2-2, the peak flow is 1990.56 m
3/s, representing a 0.62% decrease, and the peak water depth is 5.41 m, representing a 36.87% decrease. In scenario RL2-3, the peak flow is 2033.76 m
3/s, and the peak water depth is 5.55 m, representing a 1.53% increase in peak flow and a 35.24% decrease in peak water depth compared to the baseline period. In scenario RL2-4, the peak flow is 1751.75 m
3/s, and the peak water depth is 5.34 m, representing a 12.55% decrease in peak flow and a 37.69% decrease in peak water depth compared to the baseline period. In scenario RL2-5, the peak flow is 1786.75 m
3/s, and the peak water depth is 5.34 m, representing a 10.80% decrease in peak flow and a 37.69% decrease in peak water depth. In scenario RL2-6, the peak flow is 1947.12 m
3/s, and the peak water depth is 5.40 m, representing a 2.79% decrease in peak flow and a 36.99% decrease in peak water depth compared to the baseline period.
The simulation results for scenarios RL3-1 to RL3-6 are shown in
Table 12, and the outflow hydrographs are shown in
Figure 7c. The peak flow occurs between 3600 s and 5400 s for all scenarios. In scenario RL3-1, the peak flow is 2052.46 m
3/s, and the peak water depth is 5.90 m, representing a 2.47% increase in peak flow and a 31.16% decrease in peak water depth compared to the baseline period. In scenario RL3-2, the peak flow is 2088.33 m
3/s, representing a 4.26% increase, and the peak water depth is 6.21 m, representing a 27.54% decrease. In scenario RL3-3, the peak flow is 2127.37 m
3/s, and the peak water depth is 6.49 m, representing a 6.21% increase in peak flow and a 24.27% decrease in peak water depth compared to the baseline period. In scenario RL3-4, the peak flow is 1796.61 m
3/s, and the peak water depth is 5.34 m, representing a 10.31% decrease in peak flow and a 37.69% decrease in peak water depth compared to the baseline period. In scenario RL3-5, the peak flow is 1817.12 m
3/s, and the peak water depth is 5.36 m, representing a 9.28% decrease in peak flow and a 37.46% decrease in peak water depth. In scenario RL3-6, the peak flow is 2023.07 m
3/s, and the peak water depth is 5.71 m, representing a 1.00% increase in peak flow and a 33.37% decrease in peak water depth compared to the baseline period.
The simulation results for scenarios RL4-1 to RL4-6 are shown in
Table 13, and the discharge hydrograph at the outlet cross-section is shown in
Figure 7d. The peak flow occurs between 5400 s and 7200 s. In scenario RL4-1, the peak flow is 2783.00 m
3/s, and the peak water depth is 8.81 m. Compared to the baseline period, the peak flow has increased by 38.94%, and the peak water depth has increased by 2.80%. In scenario RL4-2, the peak flow is 2853.87 m
3/s, an increase of 42.48% compared to the baseline period, and the peak water depth is 8.82 m, which is an increase of 2.92%. In scenario RL4-3, the peak flow is 2974.82 m
3/s, and the peak water depth is 9.0 m. These values represent an increase of 48.52% and 5.02%, respectively, compared to the baseline period. In scenario RL4-4, the peak flow is 2156.24 m
3/s, and the peak water depth is 5.91 m. The peak flow has increased by 7.65%, while the peak water depth has decreased by 31.04% compared to the baseline period. In scenario RL4-5, the peak flow is 2289.18 m
3/s, and the peak water depth is 6.87 m. The peak flow has increased by 14.29%, while the peak water depth has decreased by 19.84% compared to the baseline period. In scenario RL4-6, the peak flow is 2694.34 m
3/s, and the peak water depth is 8.82 m. The peak flow has increased by 34.51%, and the peak water depth has increased by 2.92% compared to the baseline period.
The variations between scenarios RL1-1 to RL1-6, RL2-1 to RL2-6, RL3-1 to RL3-6, and RL4-1 to RL4-6 represent the influence of land use changes on watershed flooding. The range of variation in peak flow is 1.55% to 21.53%, 0.03% to 13.87%, 1.14% to 15.55%, and 2.55% to 27.52%, respectively. The range of variation in peak water depth is 0.56% to 37.11%, 0.00% to 3.78%, 0.37% to 17.72%, and 0.00% to 49.24%, respectively. When the rainfall gradually decreases from southeast to northwest, and the rainfall center is located downstream, the impact of land use changes on watershed flooding is significant, while in the other two land use scenarios, the impact is relatively small.
The variations between scenarios RL1-1, RL2-1, RL3-1, and RL4-1, RL1-2, RL2-2, RL3-2, and RL4-2, RL1-3, RL2-3, RL3-3, and RL4-3, RL1-4, RL2-4, RL3-4, and RL4-4, RL1-5, RL2-5, RL3-5, and RL4-5, RL1-6, RL2-6, RL3-6, and RL4-6 represent the influence of changes in rainfall characteristics on peak flow and peak water depth. The range of variation in peak flow is 3.14% to 39.85%, 4.91% to 43.37%, 4.60% to 46.27%, 2.56% to 23.09%, 1.70% to 28.12%, and 3.90% to 38.38%, respectively. The range of variation in peak water depth is 8.86% to 62.55%, 7.96% to 63.03%, 7.66% to 62.16%, 0.00% to 10.67%, 0.37% to 28.65%, and 5.74% to 63.33%, respectively. When the land use scenario is afforestation and grassland restoration, the impact of changes in rainfall characteristics on watershed flooding is relatively small, while in the other land use scenarios, the impact is more significant. Therefore, except for the land use scenario of afforestation and grassland restoration in the entire region, rainfall characteristics have a greater impact on watershed flooding than land use changes.
In short, it is stated that the most unfavorable scenario for the Hulu River Basin in terms of rainfall characteristics and land use is scenario RL4-3, with a peak flow rate of 2974.82 m3/s and a peak water depth of 9.00 m. In this scenario, the rainfall distribution is concentrated in the downstream area, and the land use corresponds to the land use in 2010. The basin is threatened by severe floods not only due to changes in rainfall distribution but also because of the reduction in grassland and forest land and the increase in cultivated land.
On the other hand, the scenario with the greatest reduction in flood risk due to rainfall characteristics and land use is RL2-4, with a peak flow rate of 1751.75 m3/s and a peak water depth of 5.34 m. In this scenario, the rainfall is heavier in the southeast and northwest regions and lighter in the northeast and southwest regions. The land use type is characterized by reforestation and the return of cultivated land to forests. The changes in rainfall distribution and the increase in grassland contribute to the decrease in flood threat.