Study On Vegetation Water Requirement Of Typical Plantation And Grassland In Loess Gully Area

Arid and semi-arid areas are sensitive areas under global climate change.As a typical semi-arid area,the gully area of the Loess Plateau is the most vulnerable area of the global ecosystem.The reality of serious water shortages makes it necessary to consider the water requirements level of the selected vegetation and whether the water requirements can be met when carrying out ecological management measures such as afforestation and grass planting in the area.At the same time,as mixed plantation begin to occupy a certain position in vegetation restoration,it is also an urgent problem to be solved to evaluate their water requirements.This study takes the Nanxiaohegou watershed,a typical small watershed in the loess gully area,as the research object.On the basis of field observations,according to the results of field observation and investigation,firstly divide the hydrological year of different rainfall years during the test period(2018,2019,2020).Then the applicability of the single and dual source ETO calculation model in the loess gully area was evaluated.Then use the dual crop coefficient principle to carry out the growth season daily-scale vegetation water demand estimation of Robinia pseudoacacia,Platycladus orientalis,Pinus tabulaeformis pure plantation and its mixed plantation of Robinia pseudoacacia-Platycladus orientalis?Robinia pseudoacacia-Pine tabulaeformis?Pinus tabulaeformis-Platycladus orientalis,Lonicera japonica Thunb plantation,natural Weeds and artificial Alfalfa grassland.Finally,while using the model to calculate the effective rainfall,the Crop Water Surplus Deficit Index is introduced to analyze the water demand satisfaction degree of different vegetation types,and an evaluation system of drought and flood indicators suitable for the loess gully area is established according to the actual situation.Finally came to the following conclusions:(1)In the single-source model analysis,the simulated value of the P-M model is used as the standard value.The unoptimized H-S model simulates too large a total amount of different hydrological years.The calculation results of Nash efficiency coefficient NSE,consistency coefficient d,and root mean square error RMSE all show that the simulation value does not meet the accuracy requirements.Optimize the H-S model through monthly linear regression,the coefficient a is distributed between-0.061 and 0.908,and the constant b is distributed between 0.318 and 3.726.The NSE of the optimized H-S model from dry years to wet years increased to 0.77,0.73,0.71;d increased to 0.93,0.92,0.91;RMSE decreased to 0.50,1.05,0.90.The deviation of the total amount from the standard value is greatly reduced to about 2%(1.70%?2.18%).The NSE,d,RMSE calculation results of the P-T model in different hydrological years show that the P-T model has reached the accuracy requirements,but the total amount of simulation values is too large.Adjust the coefficient ? of the P-T model,and the final adjustment result is a=1.122.The NSE of the optimized P-T model from dry years to wet years increased to 0.86,0.95,0.95;d changed to 0.86,0.99,0.99;RMSE was reduced to 0.31,0.18,0.38.The total deviation from the standard value does not exceed 8%(4.69%?7.85%).Overall,the simulation accuracy of the optimized P-T model is higher than that of the H-S model.(2)S-W dual-source model and P-M model basically have the same trend of change(first increase and then decrease).For evapotranspiration in the growth season,from dry years to wet years,ET0-SW were 533.29mm,703.77mm,and 754.38mm,showing that the ET0-SW is increasing year by year but the proportion is gradually decreasing.From dry year to wet year,the NSE of the S-W model is 0.52,0.57,and 0.56;its d is 0.85,0.88,and 0.89;the RMSE is 1.03,1.65,and 1.34.ET0-SW are 688.79mm,887.64mm,1050.29mm,respectively,which are 6.70%,3.75%,4.02%larger than ET0-PM.On the whole,the simulation accuracy of S-W model is not high,which means that the applicability of S-W model in loess gully area is not strong.(3)From dry year to wet year,the water requirements in June and July during the growing season is the highest,accounting for more than 40%.The water requirements of pure forest is distributed between 427.68mm?845.61mm;the water requirements of mixed plantation is distributed between 432.91mm?801.00mm;grassland is distributed between 371.2mm?730.98mm,and the water requirements of grassland is significantly less than that of f plantation.(4)In different hydrological years,the water demand of the mixed forest of Robinia pseudoacacia and Platycladus orientalis is less than that of the pure forest of Robinia pseudoacacia and Platycladus orientalis.The water requirements of the Robinia pseudoacacia-Pine tabulaeformis mixed plantation in dry years is greater than that of any pure plantation.And it is between two pure plantation in normal water years,and less than any pure plantation in wet years.The water demand of the Pinus tabulaeformis-Platycladus orientalis mixed plantation is lower than that of Platycladus orientalis and higher than that of Pinus tabulaeformis.The results of water requirements for different plantation and grassland in different hydrological years are not the same.Therefore,the frequency of the high and flat dry years in the past 50 years is used as the weight of the water requirements for different hydrological years.From big to small,they are Platycladus orientalis>Robinia pseudoacacia>Robinia pseudoacacia-Pine tabulaeformis>Pinus tabulaeformis-Platycladus orientalis>Pinus tabulaeformis>Lonicera japonica Thunb>Robinia pseudoacacia-Platycladus orientalis>Alfalfa>Weed.(5)The pure plantation show a moderately drought state in dry years,and are in a mildly drought state in normal and wet years.All mixed plantation are in a moderately dry state in drought years,and mildly drought in normal and wet years.In general,grassland has the highest degree of satisfaction with water requirements,followed by shrub plantation,and arbor plantation is the lowest.Among arbor plantation,the mixed plantation of Robinia pseudoacacia-Platycladus orientalis has the highest water requirements satisfaction degree,and Platycladus orientalis is the lowest.Among them,pure plantation shows water requirements satisfaction degree Pinus tabulaeformis>Robinia pseudoacacia>Platycladus orientalis,and mixed plantation shows Robinia pseudoacacia-Platycladus orientalis>Pinus tabul aeformi s-Pl aty cl adus orientalis>Robinia pseudoacacia-Pinus tabulaeformis.(6)On the whole,all plantations and grasslands are evaluated as drought grades at the beginning of the growing season,which indicates that the water requirements of vegetation at the beginning of the growing season is not met.According to the specific Crop Water Surplus Deficit Index,water requirements satisfaction at the end of the growth season>mid growth season>initial growth season and wet year>normal year>dry year.For vegetation restoration in the future,grass planting should be the first choice.Mixed plantations are recommended to replace pure plantations for vegetation restoration.And Robinia pseudoacacia-Platycladus orientalis mixed plantation is recommended to carry out for vegetation restoration.