| Forest cover change is important variables in the hydrological cycle and surface water quality. The vegetation, litter and soil structure in original forest land can be fundamentally changed by controlled burning before afforestation. Moreover, the water quality can be affected by managing practices such as fertilization, herbicide application, and other managements. In the other hand, the fast growing of eucalyptus can constantly change soil and vegetation structure, which strongly change the hydrological cycle and surface water quality. In this study, to investigate the eco-hydrology process and its driving force during conversion of mixed forest to eucalyptus plantation, the weather condition and surface runoff of eucalyptus plantation planted in2008(08EU) and Pinus massoniana×Cunninghamia Lanceolata,(MF) was recorded and evaporation of the sites was measured using evaporator. In addition, continuous monitoring of canopy water, surface runoff, and stream water quality was monitored on various types of planted forest [eucalyptus(02EU,06EU,08EU), Acacia mangium×A. auriculaef(AM), Pinus massoniana (PL)]. The major results from this study are:(1) Daily max air temperature, max air humility and the residual water storage space in0-50cm soil layer in eucalyptus plantation(08EU) were higher than that of mixed forest(MF) significantly, but topsoil water content and the maximum retention of canopy in08EU were lower than that of MF. Compare with MF, the fluctuations of air temperature and air humility under canopy of08EU were higher. The average min daily air humility of08EU in autumn was lower of MF by7.0%-10.6%. But no significant different of daily air temperature and humility was observed between08EU and MF. The rainfall penetration rate in canopy of08EU(92.1%) was3.8%higher than that of MF(88.7%). The max interception of08EU was8.8mm, which less than MF of6.1mm. the potential maximum retention in the0-15cm soil layer of08EU was3.7mm greater than MF, but the amount of potential maximum retention in the0-50cm soil layer and organic layer was9.5mm less than MF. Soil water content of0-30cm in08EU was lower than MF significantly, and the difference was enlarged under the condition of less rain. The mean storage of soil water in0-15cm and0-50cm layer of08EU at the period of June to December2011were15.0mm and20.0mm lower than MF, respectively, but the residual water storage space were18.6mm and10.5mm greater than MF.(2)The soil evaporation of Eucalyptus plantation was stronger than that of mixed forest, and the younger forest stand, the higher evaporation. The simulation results show that the degree of fitness in case of prediction using the paper equation and observation was higher than that of using Penman equation. The soil cumulative evaporation in11EU,08EU and MF from June2011through February2012were112.4mm,96.3mm and74.4mm,11EU and08EU exhibited38.0mm and21.9mm greater cumulative evaporation than that of MF, respectively. The peak of soil water evaporation of plantations were observed in July to September, the daily mean evaporation in the strongest month of eucalyptus planted in April2011(11EU),08EU and MF were0.786mm,0.766mm and0.579mm, respectively, it’s5.9-6.7times of that of in the lowest month(January). The higher soil evaporation generally was observed on the day after rains, and the lowest on the day before rains. The soil evaporation rates in3stands showed significant positive correlation with the daily average air temperature(Ta), daily maximum air temperature(Tm), air temperature range, air humility range(Hr) and topsoil temperature, and negative correlation with the daily minimum air humility(Hm). No significant correlation was showed between soil evaporation with soil water content. The four key meteorological factors impact on soil evaporation were daily mean air temperature(Ta), daily maximum air temperature(Tm), daily minimum air humility(Hm) and daily air humility range(Hr). The paper established a regress equation for multi-analysis of relativities between meteorological factors above mention and soil evaporation. The equation as listed below. y=0.0036Ta+0.0047Tm+0.0015Hr,-0.0013Hm+0.257The soil evaporation in08EU and MF were modeled by meteorological data. The simulation results of cumulative evaporation show the deviation of08EU and MF were-10.7%and+15.1%, respectively. The simulation results show that the degree of fitness in case of prediction using the paper equation and observation was higher than that of using Penman equation.(3)The water yield of eucalyptus plantation was less of162.3mm than that of mixed forest during June to December2011. The runoff modulus and runoff coefficient of08EU were lower than MF significantly. The strongest soil evaporation and biggest storage capacity of topsoil in eucalyptus plantation is the2major causes. The linear of storm-flow curves of08EU was similar with that of MF. The fluctuation of peak flood in08EU and MF were shortness. The cumulative flow from June through December2011of08EU and MF contributed by storm and moderate/heavy rain were75.3%,74.1%and7.0%,5.7%, respectively. The base flow in08EU was greater than that of MF significantly. The surface flow in catchment of08EU was easy occurring than that of MF during the storm event after a long time no rain. But during the following continuous storm events, the incremental speed of flood discharge in MF was greater than that in08EU. The runoff coefficient, runoff modulus and water yield of08EU were0.110,486.1m3·d-1·km-2and104.0mm, respectively, which were lower of54.4%,60.9%and162.3mm than that of MF, respectively.(4)The water quality of throughfall, surface runoff and stream flow were affected strongly by the planting of eucalyptus. The tree specie, stand age, litter and rain were the4key factors. Soil erosion strongly affected the pH and total P of surface runoff. Most of water-quality indicators of stream flow was got better during the confluence from throughfall to stream flow. The values of water color, COD and their variation of throughfall in eucalyptus plantation were significant higher than that of AM and PL, but the value of pH was lower than the latter two. The pH of throughfall in eucalyptus plantation was lower of0.7-1.0than the rainfall. Compare with different ages eucalyptus plantation, the values of water color, turbidity and COD of throughfall in the young stand were the lowest, as contrast, the middle age stand was the highest. No significant difference of total N and total P of throughfall was observed among different age forest. Surface runoff colority of mature eucalyptus plantation was3.6-5.6times and2.1times of AM and PL, respectively. The runoff pH of eucalyptus plantation was lower than PL but higher than AM. Total N concentration in runoff of young eucalyptus plantation was lower than AM and PL. However, runoff total N of gown eucalyptus plantation was higher than AM and PL. The runoff colority, pH, and COD increased with forest age in eucalyptus plantation. Runoff pH increased from4.36of young eucalyptus plantation to5.47of grown eucalyptus plantation. Runoff COD increased from28.6mg·L-1of young eucalyptus plantation to388.4mg·L-1of grown eucalyptus plantation. Runoff total N and total P were not significant different between different ages of eucalyptus plantation. Removal of litter fall significantly increased the turbidity, total N and total P in surface runoff from eucalyptus plantation, while pH, colority, and COD decreased after litter removal. Increased soil erosion after litter removal was determined as the major cause of differences. Storm events during extended duration of drought caused increased colority, COD, total N and total P, as well as decreased pH in surface runoff from planted forests. Colority and COD of stream water were significantly lower but pH higher than that of throughfall and surface runoff in eucalyptus plantation. Stream water color and pH increased during stream discharge. Forest age of eucalyptus plantation had significant effect on the stream water pH with the lowest pH observed in young plantation. Difference between water quality of stream water and the river it discharge were significantly influence by the rainfall intensity and duration between rainfall events. The pH of up cross section in PL was3.65-3.82, which was significantly lower than that of eucalyptus plantation and the big catchment river. Total N concentration in stream water from eucalyptus plantation was higher than MF significantly. No significant different of total N was observed between different sections of stream in eucalyptus catchment.(5) Total nitrogen in breakthrough water was mainly in the form of organic N, with relatively small contribution from ammonia and nitrate, but the stream water was mainly in the form of nitrate. Nitrate was the main Non-point source pollution matter in eucalyptus plantation, which mainly come from the fertilizers. Storm events are the major driving force of the non-point source of nitrogen. The younger of stand age, the more serious of N pollution. The highest concentration of total N during the Non-point source pollution was4times higher than the criterion of national standard of surface water quality(V). Ammonia was less than10%and nitrate less than3%of the total nitrogen in the throughfall. The percentage of NO-3/TN was increased with runoff volume, and it reached51.0-63.7%at the outlet of the catchment, which was significantly higher than that of MF, CL and big catchment river water. Ammonia was less than10%of the total nitrogen in the stream water from eucalyptus plantation, which was significantly lower than that of MF, CL and river water. Fertilization in eucalyptus plantation increased total N in stream water for more than20folds. The maximum total N concentration in stream water was10.6mg·L-1. Nitrate was the main form of N from fertilization of eucalyptus plantation. Storm events are the major driving force of the non-point source of nitrogen.(6) Black water in the runoff from eucalyptus plantation with high values of colority (936) and COD (136.4mg·L-1) has high concentrations of plant pigment and dissolved organic matter. Because of their short duration, blackwater events have limited ecological impact. Accumulation of eucalyptus litter on ground, high temperature, and high humidity before rainfall events were environment factors contributing to balckwater events. |
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