1. Introduction
Since the 1980s, governments around the world have taken various ecological restoration measures. The greening degree of vegetation has been increasing [
1], mainly distributed in China, India, the European Union and other regions [
2]. Large-scale afforestation has been shown to reduce river flows in the semi-arid Loess Plateau in northern China [
3] and partly contributed to the recent drought in southwest China [
4]. However, the regional dynamics of greening change are not consistent. Deforestation in Southeast Asia [
5] and the Amazon [
6] intensifies runoff and soil erosion. In addition, the steady decline of glacier area due to climate change since the beginning of the 21st century will significantly increase river flow [
7]. According to relevant studies, climate change has caused significant changes in runoff and sediment in Europe [
8], Asia [
9], America [
10] and other places. Therefore, the change in the global hydrological system caused by climate and vegetation cover has gradually become a research hotspot up to the present [
11].
The karst area in southwest China is the largest continuous karst zone in the world, and one of the regions facing the most serious soil erosion in China [
12]. Since 2002, China has carried out ecological projects in karst areas of southwest China. A large number of studies have found that the ecological recovery in karst areas with different landforms has been remarkable, the degree of stony desertification has decreased, and the per capita living standard has increased [
13]. The faulted basin area, which serves as an ecological barrier in the upper reaches of the Pearl River, has experienced a significant decline in runoff and sediment yield in recent years [
14]. However, the region has experienced dramatic changes in climate since the beginning of the 21st century, with precipitation decreasing and temperatures increasing, and notably the worst drought once in a hundred years in 2009 [
4]. Therefore, it is difficult to determine whether changes in runoff and sediment are due to ecological restoration or complex climatic changes.
Climate and underlying surfaces are considered to be the two major driving factors that lead to changes in the hydrological cycle [
15]. Precipitation and precipitation intensity directly affect the production of runoff and sediment [
16], while temperature directly changes the evapotranspiration of the surface and influences the hydrological cycle [
17]. Ecological restoration projects increase vegetation cover, and the implementation of vegetation restoration can change soil hydrological function [
18] to affect runoff and sediment yield [
19]. In addition, precipitation and temperature can indirectly affect runoff through their effects on vegetation [
20]. Therefore, the need to clearly distinguish the effects of climate and ecological engineering on runoff and sediment yield is an urgent and complex issue.
At present, the main methods are the empirical pairing method, hydrologic statistics method, hydrologic model method, neural network and other methods for studying runoff and sediment in river basins [
21]. Basin pairing requires a large amount of measured data and finding consistent meteorological conditions, which is difficult to achieve for medium and large basins [
22]. Although the neural network method can effectively simulate the relationship between runoff and sediment and corresponding factors, it is difficult to define the complex mechanism and cause of the influence [
23]. Traditional hydrological modeling methods require large amounts of data and various parameters, which will increase the uncertainty of the system [
21]. The relationship between water and heat balance based on the theoretical framework of the Budyko hypothesis has clear physical significance and easy data access, which has been widely used to attribute the change of river flow to the influence of climate and basin changes [
24,
25]. In current Budyko framework studies, the underlying surface factor
n is usually regarded as the influence of human and other non-climatic factors on runoff and sediment yield. However, attributing changes in the underlying surface factor (
n) solely to human activities may be unreasonable, as climatic factors may also play an important role in influencing this parameter [
26]. Therefore, the reasons for the change in the underlying surface factor
n include not only human factors, but also hydroclimatic variables [
27]. Jiang and Ning found that the catchment characteristic parameters were very sensitive to the combined effects of climate and artificial factors [
28,
29]. Saha et al. also proved that the assumption that the underlying surface factor
n is independent of climate variable is not valid by analytical derivation of the functional form of the Budyko equation [
30]. How to effectively separate the climate and ecological restoration of the underlying surface factor
n is a difficult problem in current research [
31].
In order to effectively distinguish the impact of climate change and ecological restoration on water and sediment change, the Budyko framework was improved in this paper, and the underlying surface factor (
n) changes were attributed to the influence of climate factors and ecological factors, and the elasticity of the two factors on underlying surface factor (
n) was calculated respectively. Sediment yield is divided into runoff and the elasticity of sediment concentration (C), which in turn is affected by land cover and climate. The elasticity of climate and ecological factors on the underlying surface factor (
n) and sediment concentration (C) was found by using the double logarithm function, so as to quantitatively calculate the influence of ecological restoration and climate change on runoff and sediment in the faulted basin [
28].
The Nandong Underground River System (NURS) has become a typical basin of the studied faulted basin due to its remarkable basin–mountain difference and complete topographic conditions. As the only outlet of the whole basin system, the south cave entrance has complete water and sediment data. In addition, the NURS is also the economic center of southern Yunnan and the seat of Honghe Prefecture. It is urgent to thoroughly reveal the evolutionary trend of climate and hydrological factors and quantitatively distinguish the contribution of climate and ecological restoration to water and sediment [
32]. Li et al. evaluated the changes of soil and water loss and sediment yield at different slope positions in the closed basin of the faulted basin, and found that the main driving factors of Cs137 concentration distribution and erosion rate were microtopographic changes, strong slope, and intensive tillage [
33]. Wang et al. found that human activities had a gradual and profound influence on the interannual characteristics of runoff in a faulted basin [
34]. Li et al. found that the decrease in rainfall not only reduced the intensity of surface soil erosion, but also reduced the flow and velocity of underground rivers, and reduced the yield capacity of suspended solids [
35]. However, before the study, the studies on water and sediment lack quantitative attribution analysis, and do not satisfactorily explain whether the reduction of water and sediment is due to climate change or ecological restoration, so it is impossible to evaluate the contribution of ecological restoration to water and sediment [
31]. Therefore, this paper selected the Nandong Underground River System as the research object, adopted the improved Budyko framework method, quantified the contribution of climate and ecological restoration to runoff and sediment, and reasonably evaluated the effect brought by ecological restoration. It provides a theoretical basis for decision makers to take corresponding measures.
3. Result
3.1. Hydrological and Meteorological Changes
The Pettitt method was used to detect the abrupt trend of runoff and sediment during the study period. As shown in
Figure 2, the runoff Pettitt value in 2002 and 2003 was the highest, reaching a significant level (
p < 0.05). The Pettitt value of 2000 and 2002 was the highest, which was very significant (
p < 0.01). Therefore, we take 2002 as the year when runoff and sediment yield in the basin changed abruptly, and 2002 is the year when ecological engineering started in the southwest karst.
Further, we set 1987–2002 as the pre-change period and 2002–2018 as the post-change period to quantitatively study the contribution of climate change and ecological restoration to runoff and sediment yield in the Nandong underground river basin system after the change. The variation trends of annual precipitation,
E0, runoff, and sediment are shown in
Figure 3. It can be found that during the variation period, precipitation, runoff, and sediment yield decrease, with sediment yield decreasing by 51.8%, runoff decreasing by 15.5%, and precipitation decreasing by 4.5%.
E0 has an increasing trend in the change period, with an increasing proportion of 1.4%, and the specific variation is shown in (
Table 2).
3.2. Sensitivity Assessment and Quantitative Analysis of Runoff Changes
The correlation between the underlying surface factor (
n) and climate change and
NDVI is shown in
Figure 4. In the NURS, the
n factor is highly correlated with
NDVI, Tmax, and Tmin, and the correlation index R
2 is 0.504, 0.418, and 0.481, respectively.
The results are shown in (
Table 3). When using
NDVI and Tmin as the double logarithm function of underlying surface factor (
n), the constant
w1 was not significant (
p > 0.05), while the significance level of
NDVI and Tmin factors (
p < 0.05), and the R
2 was 0.61. Using
NDVI and Tmax as the double logarithm function of the underlying surface factor (
n), the significance level of the constant
w1 was (
p < 0.05), while the significance level of
NDVI and Tmax was (
p < 0.01), and the R
2 was 0.61. Therefore, the study takes
NDVI as the ecological restoration factor affecting the underlying surface factor (
n), and Tmax as the climate factor affecting the underlying surface factor (
n).
According to the Budyko elasticity calculation, the elasticity and changes of ecological restoration and climate factors on runoff are shown in
Table 4. When precipitation increases by 10%, runoff increases by 18.8%; when
E0 increases by 10%, runoff decreases by 8.8%; when
NDVI increases by 10%, runoff decreases by 4%; when Tmax increases by 10%, runoff increases by 36.1%.
In the change period, the decrease in rainfall reduced the runoff by 20.5 mm, accounting for 51.2%; the increase in E0 reduced the runoff by 3 mm, accounting for 7.5%; the increase in NDVI reduced the runoff by 10.5 mm, accounting for 26.1%; the increase in maximum temperature reduced the runoff by 7.6 mm, accounting for 18.9%. To sum up, climate change resulted in a runoff reduction of 31.1 mm, accounting for 77.6%, while ecological restoration resulted in a runoff reduction of 10.5 mm, accounting for 26.1%. The calculation error is 3.6%.
3.3. Sensitivity Assessment and Quantitative Analysis of Sediment Changes
The correlation between sediment concentration C and climate change and ecological restoration is shown in
Figure 5, and the correlation coefficient R
2 between the NURS sediment concentration C and
NDVI is 0.4. Among the climatic factors, sediment concentration C has a high correlation with P and >1 AVGP, R
2 being 0.21 and 0.22, respectively. Using sediment concentration C as
y,
NDVI is a double logarithmic function of P and >1 AVGP, respectively, and the results are shown in
Table 5.
Using NDVI and P as the double logarithm function of sediment concentration C, the significance level of w2 and NDVI (p < 0.01), and the significance level of P (p < 0.05), R2 is 0.51. Using NDVI and >1 AVGP as the double logarithm function of sediment concentration C, although the significance level of NDVI (p < 0.01), the significance level of constant w2 and >1 AVGP (p > 0.05), R2 is 0.46. Therefore, the study takes NDVI as an ecological restoration factor affecting sediment concentration C, and P as a climate factor affecting sediment concentration C.
The contribution of runoff and sediment concentration to sediment yield is 19.8% and 80.2%, respectively. The elasticity of runoff to sediment yield
is 0.82 and the elasticity of sediment concentration
is 2.93. According to the Budyko elasticity calculation, the elasticity and changes of climate and ecological factors on sediment yield are shown in
Table 6. Runoff increases by 10%, sediment yield increases by 8.2%, sediment concentration increases by 10%, and sediment yield increases by 29.3%. When precipitation increases by 10%, sediment yield increases by 53.7%,
E0 increases by 10%, and sediment yield decreases by 7.2%. When maximum temperature increases by 10%, sediment yield decreases by 29.7%, and when
NDVI increases by 10%, sediment yield decreases by 32.6%.
In the change period, the decrease in precipitation led to a decrease of 13,138 t, accounting for 35.3%, the increase in E0 led to a decrease of 550 t, accounting for 7.5%, the increase in Tmax led to a decrease of 1398 t, accounting for 3.8%, and the increase in NDVI led to a decrease of 19,127 t, accounting for 51.3%. Climate change resulted in a decrease of 150,86T in sediment yield, resulting in a decrease of 40.5%, and ecological restoration resulted in a decrease of 19,127 t in sediment yield, accounting for 51.3%. The calculation error is 8.2%.