| Being the source of lives, water is an essential natural material resource for human beings. However, as a consequence of growing population and expanding industrialization worldwide, water source crisis including water shortage, pollution, and flooding is increasingly threatening human lives and sustainable development of human society. Therefore, water source investigation and protection have become a significant project for both governments and scientists. Since water crisis is virtually caused by water source management problems, effective solutions can only be found through studying management practices with a better knowledge of the nature of water source. Previous studies have shown that climate conditions and land use are the primary factors influencing water resources and water environment. Mountain areas are the source of water resources. However, because of their unique ecological fragility and higher climate sensitivity, mountainous areas are easily impacted by anthropomorphic activity and climate change effects, thus changing the distribution and flow of water resources and the quality of water ecosystems. A better understanding of effects of land use and climate changes on water resources and water environment in mountain watersheds, along with specific analysis of local watershed protection strategies (particularly mechanisms of payment for ecosystem service, known as PES), have scientific as well as practical significance concerning the improvement of watershed management and thus the alleviation of water resource crisis.In this research, a case study in Songhuaba Watershed, a vital part of Dianchi (the largest freshwater lake in Yunnan) was earried out to investigate the water resource protection issues in a typical agriculture-based mountainous watershed. With a synthesized natural science and social science methodology, we focused on the influence of climate conditions and land use on the water resources of Songhuaba Watershed. Furthermore, a questionnaire investigation on PES mechanisms in the watershed was also conducted. On the basis of our findings, water resource protection policies in Songhuaba Watershed were assessed and discussed.The main results and conclusions of this research are as follows: 1) Forest, cropland and grassland are the main land use types in Songhuaba Watershed. During the past decades, land use in this watershed has undergone dramatic changes both spatially and temporally, with overall trends of an increase in forest (17.8%) while a decrease in cropland (11.9%). From1974to1992, the watershed saw the vastest changes in land use over the whole research time range, followed by period2002-2008and1992-2002. In terms of rate of change in land use, period2002-2008was the fastest, followed by1974-1992and1992-2002. Of all the land use types, cropland went through the largest change in scale (50.0%), followed by forest (33.5%) and grassland (7.9%). However, grassland ranked the first in terms of rate of change (1.8%/a), then cropland (1.3%/a) and forest (0.6%/a). Transformations among the three land use types have been a dynamic process. Generally, increase in forest resulted from a decrease in cropland (124.4%-103.3%), while decrease in cropland was often accounted by transformation into forest (62.7%-69.5%) and grassland (20.8%-24.2%).2) Climate change was the most important factor impacting river runoff in Songhuaba Watershed, accounting for63.1%-96.6%of the total value. Land use had little impact on river runoff change as a whole, but different land use types had significantly different impacts. When rainfall was decreasing or constant, the impact of land use on a river’s runoff flow was12.2%-36.9%, whereas when rainfall increased, the impact of land use was only3.4%-5.2%. In terms of land use types, decrease in cropland or increase in grassland could lead to an increase in river runoff, while increase in cropland or decrease in grassland could result in a decrease in river runoff. However, no obvious correlation between change of forest area and river runoff was detected.3) The conversion of cropland to forest or grassland each functioned to increase river runoff (4.4%and11.3%respectively), due to decrease in water evaporation (4.8%,9.4%) and increase in base flow (14.1%,20.9%). Also, the conversion of cropland to forest and grassland affected surface runoff, but with different effects. While the conversion of croplands to forests decreased surface runoff by10.3%, the conversion to grasslands increased the value by10.2%. Our results showed that spatial pattern of forest had a direct influence on the efficiency of conversion to forest. Compared with those cropland with a slope less than5degree, conversion of cropland with a slope above5degree could significantly increase interflow (9.8%) and river runoff (4.4%) but decrease surface runoff (8.2%).4) Both land use and climate changes were primary factors influencing total nitrogen load in Songhuaba Watershed, contributing68.5%and68.2%respectively. However, our results showed that rainfall had no significant influence on total nitrogen load. Relative contribution of land use and climate changes to nitrogen load varied under different circumstances. Land use had a higher determination coefficient on total nitrogen when a dramatic change happened in the total nitrogen value (changes above40,0%); otherwise climate changes had a higher determination coefficient. Based on the correlation analysis between land use and total nitrogen load, significant correlation (0.814,-0.895) between the area of cropland and grassland and total nitrogen load was found, while no obvious correlation between forest area and total nitrogen load was detected.5) Conversion of cropland to forest, adjustment of industry structure, and water and soil protection practices could reduce total nitrogen load by38.8%,10%, and14.8%respectively. In terms of the efficiency of conversion of cropland to forest, conversion of slope cropland (with a slope more than5degree) reduced1.7times total nitrogen load than that of flat cropland (with a slope less than5degree). With regard to crop types, growing crops with high fertilization requirement generally resulted in higher nitrogen load than those with low fertilization requirement.6) In Songhuaba Watershed, investment in fertilizer took up39.7%of the total crop input. Moreover, the proportion of investment in fertilizer for food crops (61.5%) is much higher than for cash crops (44.1%). Fertilizer application varied in terms of crop types and different areas growing the same crop. According to our analysis, the following parameters tested had no significant correlation with application of nitrogen and organic fertilization of a household(R2=0.02):education level, income index, proportion of agricultural income, people force per unit cropland, area of cropland, and manure index.7) Standard (23185Yuan/hm2) for payment for ecosystem services calculated using an ecosystem service function evaluation method could be used as the upper bound of PES for conversion of cropland to forest in Songhuaba Watershed. However, PES standard (9118.5Yuan/hm2-21646.1Yuan/hm2) calculated merely by opportunity cost was not consistent and thus risky. On the other hand, data (12753.2Yuan/hm2) from the questionnaire investigation on willingness of PES could serve as reliable reference for determination of standards for PES. In Songhuaba Watershed, opportunity cost of conversion of cropland to forest (1.92×108Yuan/a) was very close to the questionnaire investigation result (2.12×108Yuan/a). By this token, a combination of these two methods could be used to make a better decision on determination of PES standard.8) Of all the PES mechanisms studied, conversion of flat cropland to forest was the most costly, with the highest PES standard (13620Yuan/hm2) and the highest payment for ecosystem services per unit (1826Yuan/kg). Conversion of slope cropland to forest had the greatest ecosystem service profit (20.1kg Nitrogen/hm2). Adjustment of industry structure did not require any payment for ecosystem services, but resulted in higher profit instead (6735Yuan/hm2). Water and soil protection practices had an advantage in terms of PES standard, which was only next to the adjustment of industry structure. However, with regard to payment for ecosystem services per unit, water and soil protection practices were only slightly better than conversion of flat cropland to forest.Novelty:1) With a synthesized natural science and social science methodology, effects of land use and climate changes on water resource were investigated. Also, distributed hydrologic model was applied to PES studies.2) The efficiency of conversion of different croplands (slope land vs. flat land) to forest was compared in terms of their influence on river runoff and total nitrogen load. PES standards and efficiency of these two practices were discussed as well. |
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