4.2. Statistical Characteristics of the Hydrological Components in Different Land Uses
The statistical result of simulation was shown in
Figure 6. Except for settlements, the total runoff of the other four land uses was similar, fluctuating between 30 and 40 mm. Wetland had the largest total runoff (39.48 mm) but was much smaller than that of the settlements (88.74 mm). The surface runoff of farmland was greater than that of the other three land uses, while the surface runoff of settlements was 6.94 times larger than that of farmland and accounted for 90.51% of its total runoff.
Subsurface runoff (lateral flow + base flow) of forests, grasslands, farmlands, and wetlands had opposite characteristics compared with surface runoff. The subsurface runoff in forests and wetlands was higher, at 26.37 and 26.87 mm, followed by grassland (23.79 mm). The subsurface runoff of farmland was lowest (17.88 mm), while the subsurface runoff of settlements was only 44.35% of that of farmland. In four land uses except for settlements, the sorting of interflow was opposite to that of base flow, which is related to the terrain conditions of different land uses.
Grassland had the highest soil water content (38.85 mm), followed by wetland (37.06 mm) and forest (34.73 mm), while the farmland’s soil water content (18.05 mm) was only 46.47% of that of grassland. The evapotranspiration of different land uses is relatively close, but the evapotranspiration of farmland is the largest, and the evapotranspiration of settlements is the smallest. Compared with farmland, forests and grasslands have less evapotranspiration and surface runoff and more subsurface runoff and soil water content, indicating that both can retain more precipitation.
4.3. Hydrological Effects of Land Use Changes
Table 4 shows the changes in hydrological components and land use areas from 2000 to 2015. The soil water content in the study area has decreased by 0.72% since 2000, and the decrease rate in 2010–2015 has an increasing trend compared with 2000–2010. The grassland area with the highest soil water content has been shrinking, shrinking by 0.99% from 2000 to 2015, and the forest area increasing first and then decreasing is the possible reason for the accelerated soil water content decrease. In 2000, 2010, and 2015, the subsurface runoff was 9.42 mm, 9.66 mm, and 9.86 mm, showing a growth trend. The growth rate of 2010–2015 (2.13%) is slower than that of 2000–2010 (2.57%). The interflow first decreased slightly (−0.07%) and then increased (+1.15%). The base flow has increased greatly, but the growth rate is reduced. The results show that the base flow is the main factor affecting the change of regional subsurface runoff, and the regional subsurface runoff’s change trend coincides with the decreased growth rate of the wetland, which had most base flow.
The surface runoff continued to increase and the growth rate continued to accelerate, where the growth rate is the fastest among all increasing hydrological components. According to the statistical results, from 2000 to 2015, 3239 ha forest, 9286 ha grassland, 33,683 ha farmland, and 290 ha wetland were converted to settlements, the more impervious surface, the more surface runoff generated. Such changes have reduced the regional soil water content, and the water source conservation ability has reduced significantly. Evapotranspiration mainly comes from the evaporation of water intercepted by the vegetation canopy and soil water, while another part comes from the transpiration of vegetation. The reduction in grassland, farmland, and forest with a rich canopy structure and underlying soil structure has caused a decline in regional evapotranspiration from 2000 to 2015.
Overall, the reduction in vegetation-covered land use and increased settlement area have ensured that regional surface runoff has comprised a larger and larger proportion of the amount of water disbursed, and this has a negative impact on the regional water conservation capacity.
4.4. Impact of Land Use Changes on Regional Water Conservation Capacity
The presented analysis shows that forests, grasslands, and wetlands could increase groundwater and have a certain water conservation capacity, but their degree of impact on the water conservation capacity is not clear. Standardized regressions of the runoff coefficient models derived using PLSR are presented in
Table 5, and the table also shows the unstandardized regression coefficients and standardized regression coefficients of each variable.
All 12 models show that the forest had a negative effect on the surface runoff coefficient, and settlements had a positive correlation with the surface runoff coefficient. Models (4), (7), (9), (10), and (12) have higher R2 and smaller p values. These five models had settlements as predictors, and the settlements’ regression coefficient is greater than 0 and the absolute value is large, demonstrating its importance to runoff generation in the study area. With an increased settlement area, the surface runoff also increased. In most models involving farmland, the standardized regression coefficient of farmland is greater than 0. The standardized regression coefficients of forests in all models were negative, indicating that forests have a negative effect on surface runoff coefficients, and forests can effectively reduce surface runoff. In most models, grassland had a negative impact on the surface runoff coefficient, but most of them were not necessary predictor variables according to cross-validation, indicating that grassland had the effect of reducing surface runoff, but its ability was weaker than that of forests.
Table 6 shows the impact of land use changes on water conservation coefficients. In all models, the standardized regression coefficient of the forest is greater than 0, and the absolute value is the largest among the land uses that have a positive impact on water conservation, indicating that forest has the strongest water conservation ability. The standardized regression coefficients of wetland and grassland are mostly greater than 0, and the standardized regression coefficients of wetland are generally greater than those of grassland, indicating that the water conservation capacity of wetlands is greater than that of grasslands. The standardized regression coefficient of farmland and settlements is less than 0, implying that increased farmland and settlement area will weaken the regional water conservation ability.
4.5. Analysis of the Forest Structure’s Influence on Conservation Capacity of Different Spatial Scales
The forest has the best water conservation capacity, and its canopy structure changes with the age of the forest and affects the forest hydrological process, in turn affecting its water conservation capacity. There are differences in the forest growth process of different forest types. Trees in the forest have a great influence on the soil water content. The analysis of the water conservation capacity changing with the age of trees in different forest types helps in formulating regional ecological restoration policies.
The hydrological effects of forest ages of different tree species are very intuitively reflected on the HRU scale. The forest canopy can intercept precipitation to reduce surface runoff, and the pore-filled soil structure can increase the soil water content, resulting in a positive effect on regional water conservation. At the same time, the forest also has high evapotranspiration. Therefore, the canopy structure, which changes with the age of the forest, affects the different hydrological processes of the forest. The amount of water conservation, evapotranspiration, and total runoff are taken as indicators to study the changes of water conservation capacity under different forest types and ages.
Figure 7 shows that the water conservation amount of deciduous conifers continued to decrease with the age of the trees. The growth of leaves in the canopy intercepted more and more precipitation, reducing the amount of water reaching the underlying surface. As a result, the amount of water conservation decreased. Starting from about the age of 25 years (young forest), the rate of decrease gradually slowed down and stabilized, and when the trees matured, the water conservation amount stabilized at approximately 25 mm. The trend of soil water conservation of evergreen conifers was similar to that of deciduous conifers, but the trend of slowing down appeared at about 40 years (medium-aged forest), and the intersection of two curves appeared at 35 years (medium-aged forest). After that, less litter and more leaves make the water conservation of evergreen conifers decreased below that of deciduous conifers. By the end of the simulation, the difference between them was 3.74 mm. During the simulation period, the average water conservation of evergreen conifers (32.57 mm) was higher than that of deciduous conifers (30.23 mm).
Evapotranspiration shows an upward trend with forest aging. During the vegetation growth, the canopy leaves increase, resulting in more trapped water. The transpiration and evaporation of the plant leaves’ trapped water increases, ultimately increasing the amount of evapotranspiration. The growth trend of deciduous conifers’ evapotranspiration was significantly weakened in about 25 years (young forest), and the growth trend of evergreen conifers was similar to that of deciduous conifers. At the age of 40 years (middle-aged forest), the total amount of deciduous conifers’ evapotranspiration exceeded that of deciduous conifers. During the simulation period, the average evapotranspiration of deciduous conifers and evergreen conifers was 306.68 mm and 304.82 mm, respectively, and the evapotranspiration ability of deciduous conifers was slightly stronger.
Total runoff decreases with increased forest aging. The reason for the negative correlation with forest age is the changing forest canopy structure. The increase in leaves reduces the chance of precipitation penetrating through the canopy, and the increased litter also holds some precipitation, thereby reducing water yield ability. The intersection of curves of different tree species’ total runoff appeared at about 30 years. After 30 years, the total runoff of deciduous conifers tended to be stable and began to be greater than that of evergreen conifers. After 30 years, the total runoff of deciduous conifers tended to be stable and began to be greater than that of evergreen conifers. Eventually, the total runoff of evergreen conifers and deciduous conifers was about 15 mm and 21 mm, respectively, and this was still in a downward trend, but the downward trend was deaccelerating.
As a constituent unit of a sub-basin, the changing hydrological components in HRUs affect the hydrological components of the sub-basin. This study changed only some of the attributes of the HRUs in a sub-basin. The variation characteristics of the hydrological components in the sub-basin may differ from those in the HRUs. Studying the hydrological effects of forest age on the sub-basin scale is more similar to the scenario of returning farmland to forest and has more practical significance.
As shown in
Figure 8, on the sub-basin scale, the changing trend of hydrological components of different forest types with the aging of trees is similar to that on the HRU scale but shows different characteristics. The intersection of the evapotranspiration curve disappeared, and the evapotranspiration of evergreen conifers on the sub-basin scale was always lower than that of deciduous conifers. There was not much difference in the evapotranspiration of different forest types after reaching maturity, and the difference was less than 1 mm. The patterns of changes in water conservation and total runoff were almost the same as those in HRUs. The patterns of changes in water conservation and total runoff were almost the same as those in HRUs. In the early stage, the two hydrological components of evergreen conifers were greater than those of deciduous conifers. At about 30 years, the relationship between the two began to change, the difference got larger, and the trend of becoming larger tended to be gentle. The long-term water conservation ability of deciduous conifers was slightly stronger.
According to People’s Republic of China Forestry Industry Standard and change pattern of curve in
Figure 7 and
Figure 8, we take trees younger than 40 as a young forest, and those older than 40 as a middle-aged forest and statistically analyze the hydrological effects of forest age and tree species changes on different spatial scale.
Table 7 demonstrates that the change of forest age significantly effects hydrological components on HRU scale, but the difference is slightly weaker on another scale. There are differences among various hydrological components of different tree species, but the differences are not statistically significant. In addition, compared with that on the HRU scale, the difference of hydrological components between different tree species on subbasin scale is more significant.