4. Discussion
4.1. Strategy of stand structure adjustment to reduce surface runoff coefficient and improve water conservation capacity of cypress forest
According to the water balance method referred in the Standard for Evaluation of Forest Ecosystem Service Function (LY/T 1721-2008) and related research (Wang et al., 2013; Si et al., 2011), when the rainfall and evapotranspiration were close, the amount of water conservation depended on the surface runoff. In this study, every runoff plots were located on the same slope and at the same altitude, and the vegetation coverage and the growth of cypress were basically the same, so the rainfall and evapotranspiration were basically the same. Therefore, the smaller the surface runoff coefficient, the higher the water conservation capacity, and reducing the surface runoff coefficient would indirectly improve the water conservation capacity of cypress forest.
According to the coupling effects of the spatial distribution of cypress and topography on surface runoff coefficient, through the adjustment of the stand structure, the system structure of runoff generation in the terrain unit would be changed, thereby reducing the surface runoff coefficient and improving the water conservation capacity of cypress forest.
The actual observations of the runoff plots showed that the surface runoff coefficient of the study area was concentrated in the range of 0.3 to 0.4. When the surface runoff coefficient was large (M>0.5), the main characteristics of the spatial distribution of cypress could be divided into two categories. The first type had low stand density of cypress (T<20 ind/100m2) and regular spatial structure (W<0.5), and the second type had moderate stand density of cypress (20 ind/100m2< T<50 ind/100m2) and regular spatial structure (W<0.5). In both cases, the composite index of topography was relatively high (U>12). According to the interaction coefficients between the independent variables determined by the SEM, the response of the composite index of topography to the spatial distribution of cypress could be determined. Combining the response regression equation of surface runoff coefficient to the composite index of the spatial distribution of cypress and the composite index of topography, the dynamic change curve of surface runoff coefficient with the change of the spatial distribution of cypress was constructed (Fig.14, Fig.15).
For different spatial distribution of cypress, two strategies of stand structure adjustment could be adopted to reduce surface runoff coefficient (The designed target surface runoff coefficient was within 0.3), including A) only increasing the stand density of cypress, or B) increasing both the stand density and the contagion index of cypress (The strategy of only increasing the contagion index of cypress shown in the figure could not achieve the goal of reducing the surface runoff coefficient to 0.3).
In the case of low stand density and regular spatial structure of cypress (Fig.14), the surface runoff coefficient could be reduced from 0.62 to less than 0.3 by A) increase the stand density of cypress from 14 ind/100m2 to 24 ind/100m2(corresponding to the composite index of topography reduced from 17.4 to 9.1) or B) increasing the stand density of cypress from 14 ind/100m2 to 21 ind/100m2 and the contagion index from 0.41 to 0.56 (corresponding to the composite index of topography reduced from 17.4 to 8.0). Comparing strategy A and strategy B in fig.24, it could be found that when the stand density of cypress was increased to the same value, strategy B reduced the surface runoff coefficient by 4%~6.8% higher than strategy A.
In the case of moderate stand density and regular spatial structure of cypress (Fig.15), the surface runoff coefficient could be reduced from 0.57 to less than 0.3 by A) increase the stand density of cypress from 36 ind/100m2 to 64 ind/100m2(corresponding to the composite index of topography reduced from 12.3 to 6.0) or B) increasing the stand density of cypress from 36 ind/100m2 to 52 ind/100m2 and the contagion index from 0.46 to 0.58 (corresponding to the composite index of topography reduced from 12.3 to 6.3). Comparing strategy A and strategy B in fig.26, it could be found that as the stand density of cypress increased, the difference in the reduction of surface runoff coefficient between strategy B and strategy A gradually expanded. When the stand density of cypress increased to 55 ind/100m2, strategy B could reduce the surface runoff coefficient by 23.9% higher than strategy A.
Comparing the reduction efficiency of the surface runoff coefficient by strategy A and strategy B under the two cases, it could be found that when the stand density of cypress was low (T<20 ind/100m2), increasing the contagion index of cypress on the basis of increasing the stand density of cypress to a certain index could not significantly improve the reduction of surface runoff coefficient. However, when the stand density of cypress was moderate (20 ind/100m2< T<50 ind/100m2), increasing the contagion index of cypress on the basis of increasing the stand density of cypress to a certain index could greatly improve the reduction of surface runoff coefficient. Therefore, for the strategy of stand structure adjustment in the study area, when the initial stand density of cypress was relatively low (<20 ind/100m2), the first step was to increase the stand density of cypress, and until the stand density of cypress reached to moderate level (20-50 ind/100m2), adjusting the spatial structure of cypress from relatively regular to relatively clumped could reduce the surface runoff coefficient to a greater extent.
4.2. Selection of key factors when studying the coupling effects of the spatial distribution of cypress and topography on surface runoff coefficient using SEM
The SEM used in this study was a confirmatory model, not an exploratory model. It required the support of theory or empirical rules to construct a hypothetical model, and the consistency of the theoretical model and the actual observation was tested through sampled data. When using SEM to study the coupling effects of the spatial distribution of cypress and topography on surface runoff coefficient, the theoretical basis was that different spatial distribution of cypress were distributed in slope units with different topographical features, which changed the system structure of runoff generation in the unit, enhanced or weakened the water blocking capacity of the landscape system, thereby changing the distribution and intensity of surface runoff (Slattery and Burt, 2015).
In the selection of key factors, on the one hand, it was necessary to exclude the interference of factors other than characteristic parameters of the spatial distribution of cypress and topography. In this study, the disturbing factors that could have an impact on surface runoff coefficient included rainfall intensity, surface vegetation coverage, the relative height difference, etc. Therefore, in the process of experimental design and data collation, the above disturbing factors should be controlled or explained. On the other hand, the selected key factors should be able to fully reflect the characteristics of the spatial distribution of cypress and topography, while having a significant impact on surface runoff coefficient. Therefore, based on a large number of studies on the influencing factors of surface runoff, seven common independent factors were selected, and the correlation between each factor and surface runoff coefficient was analyzed, and finally five factors that had significant effects on surface runoff coefficient were screened out as independent variables for SEM construction. The theoretical model constructed therefrom had a high degree of fit and could simulate the causal relationship and coupling mechanism among the spatial distribution of cypress, topography and surface runoff coefficient.