The Evapotranspiration Of Typical Vegetation And The Scale Effect Of The Hydrologic Features In Slopes Of Diediegou Watershed Of Liupan Mountains

Low coverage of vegetation and harsh environment severely limit the economic and socialdevelopment of the regionsin northwest China. Especially in the arid and semi-arid areas,it isthe key to rebuild the vegetation for the purpose of eco-enviroment improvement. In the pastwork of vegetation restoration, there existedmany mistakes, such as selectingunfitvegetation,afforestation in unsuitable area or with astand densitylager than the carryingcapacity of the soil water. In fact, limited water resources and its strong heterogeneityallocating in space determinein a great degreethe vegetation type, pattern and structure. Toprovide a reasonable basis for vegetation restoration, it is necessary to understand the variationand scale effect of topography, soil thickness and water content and other site factors in space,and the current distribution and variation of vegetation structure and scale effect, as well as therelationship between vegetation and site factors. To this end, we selected a series of slopes andplots in a semi-arid region northwest of the Liupan Mountains, to study the temporal andspatial variation of evapotranspiration and its components in typical vegetation, and to quantifythe slope variation and spatial scale effect of the hydrological factors, such as the siteconditions and forest structure,etc. The main conclusions are as follows:1. Difference in sap flow density among Larix principis-rupprechtii Mayr trees ofdifferent degree of dominanceThe sap flow density (Js) in trees of higher degree of dominance started earlier in themorning but ended later at night than that in trees of lower degree of dominance. In addition,the maximum of Jsduring the daytime appeared earlier and was higher in higher trees. Thus thedaily average of Jsin higher trees was apparently higher as well. Furthermore, the Jsin highertrees was more sensitive to the solar radiation and the vapor pressure deficit on a5-min timescale, while on daily scale to the soil drought, higher trees showed less sensitive, implying astronger ability for obtaining light and water. In general, however, no significant difference was found in the pattern of Jsresponse to environment conditions among sample trees, and therelative difference in Jswas relatively stable. Correlation analysis indicated that the mostimportant factors positively affecting the Jswere the degree of dominance (relative height) andthe tree height (P＜0.01), then were the canopy length and diameter at breast height (P＜0.05),and then the canopy width and sapwood area (P＞0.05). In an improved approach, the Jsforthe forest stand was taken as the average of Jsfor all individual trees in the stand which wascalculated by using the linear relation between Jsand degree of dominance. It was16%lessthan the value calculated by the average of Jsfor the five sample trees in the widely used basicapproach. Therefore it is proposed that the degree of dominance should be taken into accountin the up-scaling approach for the stand Jsor transpiration estimate in future.2.Coupling effects of soil moisture and evaporative demandon the larch transpirationDaily transpiration(T)of the larch plantation varied from0.08to2.18mm·day-1for thewholegrowing season.In any given soil watercondition, the T displayed scattered, andmuchmore scattered in better conditions, showing the coupling effects of evaporative demand(PET) and soil water (REW). A logistic relationship(T=Tmax·(1-exp(k·REW)) was derived todescribe the T varying with the rising REW. Additionally, theTmax(the maximum of T under agiven PET condition) increasedwith the PET (Tmax=a·PET2+b·PET). After combining these twofunctions together, a more general model covering the whole variation range of PET and REWwas determined as T=(-0.05·PET2+0.65·PET)·(1-exp(-5.39·REW). This model fitted themeasured data well and could explain92%of the T variation in the larch plantation, and thuscan be used to describe the joint influence of REW and PET on T.3. Characteristics of evapotranspiration and water balance in several kinds of vegetationCanopy interception were different among various kinds of typical vegetation. During theperiod of July to October,2012,with a cumulative precpitation of366mm, the interception washighest in the birch forest (15%of the precipitation),then wasthe sea buckthorn shrubs, larchplantation, mountain peach plantation and miscellaneous shrubs(11-12%,40-44mm), thelowest9%(31mm)was the ostryopsis shrubs. The difference of the evapotranspiration under the canopy (38.64-49.84mm per month)among vegetation was lower in the beginning of the growing season, but higher in the peakperiod,60.59-104.93mm per month. Obvious change of the evapotranspiration was found inthe sunny slope grassland, mountain peach plantation,ostryopsis shrubsand semi-sunny slopegrassland, whereas not in the larch plantation or the sea buckthorn shrubs. Temperature was themost influential factor, then was the potential evapotranspiration, solar radiation and humidity.The canopy interceptionwas18%,12%of the total evapotranspiration in the growingseason of2011and2012, respectively in the larch plantation. The tree transpirationtookaproportion of25%,22%; meanwhile the evapotranspiration,57%,65%, respectively. Theseasonal change of the ratio of each component to the total evapotranspiration was different.The ratio of interception changed from0to32%,6%to24%, in2011and2012, respectively,with a comparatively smooth tendency; the ratio of transpiration changed from6%to51%,7%to33%, with a gradual decrease; however the ratio of theevapotranspiration under the canopychanged from39%to69%,53%to8%, with a gradual increase during the growing season.The cumulative amount of precipitation was424,526mm in2011and2012, respectively.The evapotranspiration of the larch plantationaccount for106%,103%of the precipitation; thewater yield was16.4,165.3mm and external inputs of water for use was126.4,12.2mmin2011and2012, respectively. The evapotranspiration of sunny slope grasslandaccounted for73%of the precipitation;the water yield was111.4,238.2mm. The evapotranspiration ofsemi-sunny slope grasslandaccounted for70%of the precipitation in2011,68%in2012andthe water yield was139.0,259.9mm, respectively.4.Slope variation and scale effect of the forest structureObvious distinctions were found in the canopy leaf area index (LAI) and abovegroundbiomass among slope directions and positions. The averagesof LAI and aboveground bimasswere higher on the shady slopethan on the semi-shady slope.And along the slope, both of themperformed an increase with the position decreasing,whichcould bewell related tothe soil depthor moisture conditions in different slope positions. Scale effect occurred in the slope variation of the forest stucture. Theslope average ofcanopy LAI increased0.22per100mincrease in the slope length on the shady slope, whichwas higher than0.16on thesemi-shady slope. Therefore, the scale effect of the LAI wasstronger on the shady slope. The slope average of aboveground biomass increased4.92t·hm-2per100m increase in the slope length on the shady slope, which was lower than6.28t·hm-2on the semi-shady slope, implying a stronger scale effect on the semi-shady slope.Furthermore, obvious differencewas obeseverd in the values measured in different slopepositions for representatingthe entire slope. Relationships between the ratio of plot value toslope average value of the canopy LAI or aboveground biomass with the horizontal distancefrom the slope top was fitted.Based on this, values of vegetation structure obtained at a specificposition of a slope can be translated into an average for the entire slope.It is very helpful fortheefficiency and accuracy of the slope parameter estimates.5. Slope variation and scale effect of the forest evapotranspiration and water yieldObvious distinctions were found in the forest evapotranspiration (ET) and water yield (Y)among slope directions and positions. The ET was higher on the shady slope than on thesemi-shady slope, whereas the Y was lower on the shady slope. With thepositon decreasingalong the slope, the ET showed a gradual increase, whereas the Yperformed in a converse way.The scale effect of the ET was stronger onthe shady slope (28.3mm/100m, i.e. the slopeaverage of forest ET increased28.3mm per100m increase in the slope length) than on thesemi-shady slope (19.5mm/100m). In contrast, the scale effect of the Y was weaker on theshady slope (16.8mm/100m) than on the semi-shady slope (34.7mm/100m).Relationshipsbetween the ratio of plot value to slope average value of the ET or the Y with the horizontaldistance from the slope top was fitted.Based on it, the slope average can be estimated upscalingfrom the value measured on any one specific plot on the slope.