| Influenced by the slope difference of hydrological factors such as vegetation and soil,the hydrological processes often have a large spatial?slope position and slope length?and temporal?month?variations on the slope.Past studies have focused on the plot and watershed scale,and given less attention to the critical scale of slope,which limits the understanding of the formation mechanism of scale effect and the transition of the results at different scales.To understand the spatio-temporal variation mechanism of hydrological impacts at slope scale and improve the accuracy of slope hydrological impacts evaluation,a representative slope of Larix principis-ruprechtii plantation with a horizontal length of 425.1 m was selected in the small watershed of Xiangshuihe of Liupan Mountains in northwest China.A 30 m wide survey transect was set up from slope top to slope foot on this slope,and it was evenly divided into 16continuous plots.The dynamics of vegetation structure,and soil hydro-physical properties were monitored;and meanwhile,the related hydrological processes were synchronously measured.The spatial-temporal variations of influening factors and their effects on a series of hydrological processes were analyzed.The dominant factors causing the variations of hydrological processes along the slope were determined.The approach of up-scaling from a plot to the whole slope was explored for the influencing factors and hydrological processes.Using the absolute values of the mean change of the downwards moving average of interested parameters within the horizontal distance of 100 m,the slope length effect of the influencing factors and hydrological processes within slope scale was evaluated.1.Spatial-temporal variations of vegetation characteristics along the slopeThe stand aboveground biomass showed a trend of firstly gradual increase along lowering slope positions,reaching its maximum at the middle-upper slope position,and thereafter a gradual decrease.The variation trend of all the components of aboveground biomass?trees,shrubs,herbs and litter?was similar to that of the aboveground biomass.The soil moisture was the dominant factor to determine the spatial variation pattern of aboveground biomass on the study slope,showing a significant positive correlation among the soil moisture and aboveground biomass.The moving average of the aboveground biomass showed an increase firstly and then decrease with increasing slope length,with the slope length effect of+2.89t?hm-1/100 m?0–244.2 m?and-2.46 t?hm-1/100 m?244.2–425.1 m?.The stand LAI had a remarkable variation along the slope with a remarkable difference among the months.During the whole growing season,the variation of LAI along lowering slope positions presented a trend of firstly increase,reaching its maximum at the middle slope,and then decrease.In May,the LAI showed a decreasing tendency along lowering slope positions.In June,July and August,the LAI presented a trend of firstly increase,reaching its maximum at the middle slope,and then decrease along lowering slope positions.In September and October,the LAI showed an increase trend along lowering slope positions.The seasonal variation of main influencing factors caused the above mentioned temporal variation of slope position difference of LAI.In May,the main influencing factor was thought to be the difference in solar radiation and temperature among slope positions caused by terrain shade.In the period from June to August,the main influencing factor should be the soil moisture limit.In the period of September and October,the joint impacts of topography?elevation?slope position?,slope gradient?,meteorological condition,soil moisture and soil hydrological properties led to the observed variation of LAI along slope position.During the whole growing season,the moving average of LAI showed a“increase-decrease”tendency with increasing slope length,with the slope length effect of+0.07/100 m?0–296.7 m?and-0.02/100 m?296.7–425.1 m?.In May,the moving average of LAI showed a decreasing tendency with increasing slope length,with a slope length effect of-0.02/100 m.In June,July and August,the moving average of LAI showed an increase firstly and then decrease with increasing slope length,with a slope length effect of+0.15/100 m,+0.16/100 m and+0.18/100 m?0–244.2 m?,and-0.09/100 m,-0.08/100m and-0.07/100 m?244.2–425.1 m?,respectively.In September and October,the moving average of LAI also showed an increase trend along slope length,with slope length effect of+0.03/100 m and+0.09/100 m,respectively.2.Spatial variation of soil hydrological properties along the slopeThe soil hydrological properties had remarkable differences along the slope,but the variation of each soil physical indexes in 0–100 cm soil layer showed different variation trends along lowering slope positions,“decrease-increase-decrease”for soil bulk density,“increase-decrease-increase”for saturated water-holding capacity and total porosity,“increase-decrease”for field capacity and capillary porosity as well as saturated hydraulic conductivity,and“decrease-increase”for non-capillary porosity,respectively.The vegetation structure?stand density?and topographical factors?altitude and gradient?were the dominant factor determining the spatial variation of soil physical properties on the study slope,showing a significant negative correlation between the soil bulk density and stand density,a significant positive correlation between soil non-capillary porosity and stand density,a significant positive correlation between soil saturated water-holding capacity and porosity and altitude,and a significant negative correlation between the soil field capacity,capillary porosity and saturated hydraulic conductivity and slope gradient.The moving average of soil bulk density,field capacity showed“decrease-increase-decrease”trend with increasing slope length.The moving average of capillary porosity,non-capillary porosity,total porosity,saturated water-holding capacity and saturated hydraulic conductivity showed“increase-decrease”trend with increasing slope length.The slope length effect varied in different soil physical indexes,soil bulk density was-0.05 g?cm-3/100 m?0–134.9 m?,+0.02 g?cm-3/100 m?134.9–351.9 m?,and-0.01 g?cm-3/100 m?351.9–425.1 m?;saturated water-holding capacity was+0.27%/100 m?0–179.3 m?and-0.96%/100 m?179.3–425.1 m?;field capacity was-0.94%/100 m?0–77.9 m?,+1.16%/100 m?77.9–301.8 m?and-2.60%/100 m?301.8–425.1 m?;capillary porosity was+0.54%/100 m?0–314.6 m?and-0.61%/100 m?314.6–425.1 m?;non-capillary porosity was+0.78%/100 m?0–94.7 m?,-0.60%/100 m?94.7–364.4 m?,and+0.58%/100 m?364.4–425.1m?;soil porosity was+0.76%/100 m?0–143.6 m?and-0.52%/100 m?143.6–425.1 m?;saturated hydraulic conductivity was+0.04 mm?min-1/100 m?0–170.1 m?and-0.02mm?min-1/100 m?170.1–425.1 m?.There was a remarkable variation in soil moisture along the slope.During the whole growing season,soil moisture on the slope showed a trend of firstly a gradual increase from slope top with increasing slope length,reaching its maximum at the middle-upper slope position?at the horizontal slope length of 135.8 m?,and thereafter a gradual decrease.The variation trend of soil moisture from May to September was similar to that of the whole growing season.Soil moisture in October showed an decrease trend along lowering slope positions.This difference may be due to the freezing soil in October,which is not conducive to the redistribution of soil water.Under the study year with relative rich precipitation,the soil water-holding capacity became the dominant factor to determine the spatial variation pattern of soil moisture on the study slope,showing a significant correlation among the soil moisture and the soil porosity and water-holding capacity in every month.Meanwhile,litter biomass also had an important effect on the soil moisture for the months?June,July,and September?with relative rich precipitation.The moving average of soil moisture during the growing season and every month has different degrees of change with increasing horizontal slope length,and thus the soil moisture showed a slope effect.For the whole growing season,May,June,and July,the slope length effect was+0.61%/100 m,+1.18%/100 m,+1.38%/100 m and 0.46%/100 m?0–217.6 m?,and-0.82%/100 m,-0.95%/100 m,-0.92%/100 m and-0.82%/100 m?217.6–425.1m?.The slope effect was+0.47%/100 m?0–171.1 m?and-0.52%/100 m?171.1–425.1 m?in August,+0.98%/100 m?0–160.2 m?and-0.48%/100 m?146.2–425.1 m?in September,-0.88%/100 m?0–425.1 m?in October.3.Spatial-temporal variations of canopy interception along the slopeThe canopy interception presented a remarkable variation along the slope with a remarkable difference among the months.During the whole growing season,the overall variation tendency of canopy interception along lowering slope positions showed firstly an increase,reaching its maximum at the middle slope,and then a decline.In May,the variation of canopy interception along lowering slope positions showed a decreasing tendency.In June,July and August,the variation tendency of canopy interception along slope length in these months was consistent with the whole growing season.In September and October,the variation of canopy interception along lowering slope positions showed an increasing tendency.The LAI was the main factor affecting the spatial-temporal variations of canopy interception.The correlation between canopy interception ratio and LAI was positive and significant in every month.During the whole growing season,the moving average of canopy interception showed a trend of“increase-decrease”with increasing slope length,with the slope length effect of+5.62mm/100 m?0–316.6 m?and-2.37 mm/100 m?316.6–425.1 m?.In May,the moving average of canopy interception showed a decrease trend with increasing slope length,with a slope length effect was-0.26 mm/100 m?0–425.1 m?.In June,July and August,the moving average of canopy interception showed a trend of“increase-decrease”with increasing slope length.In June,the slope length effect was+1.28 mm/100 m?0–261.1 m?and-1.78 mm/100 m?261.1–425.1 m?.In July,the slope length effect was+0.92 mm/100 m?0–267.6 m?and-0.88 mm/100m?267.6–425.1 m?.In August,the slope length effect was+1.28 mm/100 m?0–211.2 m?and-0.34 mm/100 m?211.2–425.1 m?.In September and October,the moving average of canopy intrception showed an increase trend with increasing slope length,with the slope effect of+2.38 mm/100 m and+0.81 mm/100 m?0–425.1 m?,respectively.4.Spatial-temporal variations of stand transpiration along the slopeSlope stand transpiration are jointly influenced by potential evapotranspiration?PET?,relative soil water content?REW?and canopy LAI.The function types of stand transpiration response to PET,REW and LAI were determined based on measured data and boundary line.Stand transpiration responds to PET conformed to the binomial equation,and respond to REW in different soil layers and canopy LAI conformed tothe saturated exponential growth equation.A stand transpiration model considering the effect of the demand of atmospheric evaporation?PET?,water supply capacity of soil?REW?and stand characteristic?canopy LAI?was fitted using multiplicative equation:T=?-1.151PET2+9.191PET-3.271?×[-7.170+0.335(1-EXP(-5.913REW0-10cm))+7.066(1-EXP(-11.813REW10-20cm))+21.333(1-EXP(-14.493REW20-40cm))+0.754(1-EXP(-8.180REW40-60cm))]×[-0.043+0.048?1-EXP?-1.681LAI??].This model can well evaluate the variation of daily stand transpiration.Stand transpiration had a remarkable variation along the slope.During the whole growing season,the overall variation tendency of stand transpiration along lowering slope positions showed firstly an increase,and then a decline.In May,June,July,and August,the variation tendency of stand transpiration was consistent with the whole growing season.In September and October,stand transpiration showed an increasing tendency along lowering slope positions.Canopy LAI,soil moisture and the changes in meteorology caused by altitude were the main factors affecting the spatial-temporal variations of stand transpiration.The correlations between stand transpiration and canopy LAI and soil moisture were significant in May and Jun..The correlations between stand transpiration and canopy LAI were significant in July,August,and September.The correlations of stand transpiration with canopy LAI,soil moisture,altitude,gradient and PET were significant.During the whole growing season,the moving average of stand transpiration showed a trend of“increase-decrease”with increasing slope length,with the slope length effect of+7.87 mm/100 m?0–303.3 m?and-2.52 mm/100 m?303.3–425.1 m?.In May,June,July,and August,the variation tendency of moving average of stand transpiration along slope length was also consistent with the whole growing season.In May,the slope length effect was+2.64mm/100 m?0–293.5 m?and-1.00 mm/100 m?293.5–425.1 m?.In June,the slope length effect was+1.75 mm/100 m?0–283.4 m?and-0.96 mm/100 m?283.4–425.1 m?.In July,the slope length effect was+1.07 mm/100 m?0–276.8 m?and-0.67 mm/100 m?276.8–425.1 m?.In August,the slope length effect was+0.97 mm/100 m?0–307.4 m?and-0.42 mm/100 m?307.4–425.1 m?.In September and October,the moving average of stand transpiration showed an increase trend with increasing slope length,with the scale effect of+0.62 mm/100 m and+0.43 mm/100 m?0–425.1 m?,respectively.5.Spatial-temporal variations of forest floor evapotranspiration along the slopeSlope forest floor evapotranspiration are jointly influenced by potential evapotranspiration?PET?,soil water content?SMC?and canopy LAI.The function types of forest floor evapotranspiration response to PET,SMC and LAI were determined based on measured data and boundary line.Forest floor evapotranspiration responds to PET conformed to linear equation,responds to SMC conformed to the saturated exponential growth equation,and responds to canopy LAI conformed to the exponential decay equation.A forest floor evapotranspiration model considering the effect of the demand of atmospheric evaporation?PET?,water supply condition of soil?SMC?and canopy cover?canopy LAI?was fitted using multiplicative equation:E=?6.6971PET-2.7704?×?6.9274-11.2434EXP?-1.9588SMC??×?0.0324+1.1622EXP?-2.3069LAI??,and the forest floor evapotranspiration estimated by micro-lysimeter was further adjusted based on this model and the observed soil moisture.Forest floor evapotranspiration had a remarkable variation along the slope with a remarkable difference among the months.During the whole growing season,forest floor evapotranspiration showed an increasing tendency along lowering slope positions.In May,Jun.,Jul.,the overall variation tendency of forest floor evapotranspiration with rising horizontal slope length showed firstly an unchanging,and then an increase.In August and September,forest floor evapotranspiration showed an increasing tendency along lowering slope positions.In October,the overall variation tendency of forest floor evapotranspiration along lowering slope positions showed firstly an increase,and then a decline.The main factors affecting the variation of forest floor evapotranspiration along the slope had a remarkable difference among the months.Canopy LAI,the changes in meteorology caused by altitude and slope gradient and litter biomass were the main factors causing the variations of forest floor evapotranspiration along the slope in May,June and July.The changes in meteorology caused by altitude and slope gradient was the main factor for causing the variations of forest floor evapotranspiration along the slope in August.The changes in the meteorology caused by altitude and litter biomass were the main factors causing the variations of forest floor evapotranspiration along the slope in September.Complex meteorology caused by alititude and terrain shade were the main factors causing the variations of forest floor evapotranspiration along the slope in October.For the whole growing season,May,June,July,August,September,and October,the moving average of forest floor evapotranspiration showed an increase trend with increasing slope length,with the slope length effect of+5.88 mm/100 m,+0.86 mm/100 m,+0.89 mm/100 m,+1.30 mm/100 m,+0.72 mm/100 m,+0.83 mm/100 m and+1.28 mm/100 m?0–425.1 m?,respectively.6.Spatial-temporal variations of stand water yield along the slopeMaintaining a certain water yield is the basic requirement for the management of water-retention forests,and it needs to be accurately estimated.Stand water yield had a remarkable variation along lowering slope positions and with a remarkable difference among the months,a“decline-increase”for the whole growing season and for June,“increase-decline-increase”for July and August,“decline-increase-decline”for September,and“decline”for October.The main processes and factors affecting the variation of stand water yield along the slope had a remarkable difference among the months.In June,canopy interception,slope grandient and soil water-holding capacity were the main hydrological process and factors causing the variation of water yield along the slope.Soil water storage and infiltration ability?and also the runoff from upper slope for September?were the main hydrological process causing the variation of water yield along the slope in July,August,and September.In October,canopy interception,canopy LAI,soil infiltration ability,and soil porosity were the main influence factors causing the variation of water yield along the slope.During the growing season,the moving average of stand water yield showed a decrease trend with increasing slope length,with the slope length effect of-10.3 mm/100 m?0–425.1 m?.In June,the moving average of stand water yield showed a“decline-increase”trend with increasing slope length,with the slope length effect of-4.8 mm/100 m?0–296.2 m?and+2.8mm/100 m?296.2–425.1 m?.In July,the moving average of stand water yield showed a“increase-decline”trend with increasing slope length,with the slope length effect of+2.9mm/100 m?0–186.9 m?and-2.6 mm/100 m?186.9–425.1 m?.In August,the moving average of stand water yield also showed a“increase-decline”trend with increasing slope length,with the slope length effect of+6.6 mm/100 m?0–175.8 m?and-2.3 mm/100 m?175.8–425.1 m?.In September,the moving average of stand water yield showed a“decline-increase”trend with increasing slope length,with the slope length effect of-12.8 mm/100 m?0–258.0 m?and+1.7mm/100 m?258.0–425.1 m?.In October,the moving average of stand water yield showed a decrease trend with increasing slope length,with the scale effect of-2.5 mm/100 m?0–425.1m?.7.Upscaling of hydrological factors and stand evapotranspiration from plot to slopeTo improve the evaluation accuracy of slope hydrological factors,the statistical relations reflecting the variation of the ratios of aboveground biomass and LAI at different slope positions to the whole slope average with increasing horizontal slope length from slope top were fitted.Using these relations,the whole slope average can be estimated from the measured value at any slope position.The best representative slope position for the whole slope average of soil moisture was found at the middle slope position?corresponding to the relative horizontal slope length of 0.57?.Therefore,the observed soil moisture at this slope position can be simply viewed as the slope average.Stand evapotranspiration and its components have scale effect between plot and slope.Thus,it is difficult to directly evaluate the slope value using the survey data of representative plots.A method evaluating the slope stand evapotranspiration was developed.The slope canopy interception could be estimated through the up-scaling from the measured LAI at certain plot,the relation between plot LAI at different slope positions and the slope average of LAI,and the relation between canopy interception and stand LAI.The slope average of stand transpiraton?and forest floor evapotranspiration?could be estimated through the up-scaling from the measured LAI and REW?SMC?at certain plot,the relation between plot LAI at different slope positions and the slope average of LAI,the transition of plot soil moisture to the slope average of soil moisture,and the relation between stand transpiration?and forest floor evapotranspiration?and slope PET,REW?SMC?and canopy LAI.After summing up the up-scaling estimation values of evapotranspiration components,the slope stand evapotranspiration can be estimated.The slope stand water yield could also be further estimated using the water balance formula. |
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