Influence Of Populus×Euramericana Plantation On Rainfall Redistribution And Soil Water Movement Under Different Rainfall Patterns

As a common tree species for ecological restoration in northern China,the poplar plantation promotes rainfall infiltration and water conservation and has an important ecological service function over ecological hydrological cycles.Therefore,we took the Populus×euramericana plantation ecosystem as the research object.Long-term field observations were carried on the canopy hydrological process,atmospheric environmental factors and soil moisture,etc.We adopted cluster analysis to analyze the rainfall pattern,and established a GASH model for quantitative analysis of the canopy rainfall redistribution process.The Hydrus-1D porous solute transport model was used to simulate the soil water transport process during the same period,and the observation data were employed to validate the model simulation results.We aim to evaluate the rainfall redistribution process of Populus×euramericana plantations under different rainfall patterns and their effects on soil water transport.The results show:（1）During the experiment period,77 rainfalls were observed,with a total amount of 665.9 mm,and the number of rainy days reached 89 days.The peak of rainfall was reached in summer.According to the rainfall amount,duration,average rainfall intensity and the first 30 min rainfall intensity,the rainfall types were divided into four categories.Type I rainfall was low rainfall（average 2.18mm）,short duration（average 197.94min）,light rain intensity（average 1.41mm/h）,classified as light rain.Type II was extremely high rainfall（116.80mm）,long duration（2649.60）min),high rain intensity（2.64mm/h）,classified as heavy rain.Type III was medium rainfall amount（mean 11.44mm）,medium duration（mean527.38min）,medium rain intensity（mean 1.76mm/h）.Type IV was relatively high rainfall（mean23.55mm）,medium duration（mean 518.15min）,high rain intensity（mean 7.10mm/h）.Type III and IV were classified as moderate rain.（2）The throughfall accounted for 67%of the total rainfall in the same period,which was significantly related to the rainfall outside the forest.Among them,Type II and Type IV rainfalls contributed significantly to the throughfall,accounting for 40%and 37%of the throughfall,respectively.The throughfall rate of Type II rainfall and Type III rainfall reaches 70%.Under different rain types,the characteristics of throughfall delayed relative to beginning of the rainfall events.Under the Type II rainfall,throughfall has the shortest delay time（6.52 min）at the beginning compared to other types,and it ended earlier than the rainfall,resulting in shortened rainfall duration under the canopy.The throughfall were delayed in the beginning and ending stages under all other rainfall types.Under all rainfall types,the delay time at the beginning stage is significantly negatively correlated with the average rain intensity the first30 minutes rainfall intensity.The delay time is significantly negatively correlated with the rainfall duration,and was correlated positively to the average rain intensity.The throughfall duration was negatively correlated with the rainfall duration,and positively correlated with the average rainfall intensity.（3）The stemflow accounted for 7%of the total rainfall,which was significantly positively correlated with the rainfall amount and rainfall intensity.Type IV rainfall had a higher contribution to stemflow,accounting for 48.1%of stemflow.Under Type IV rainfall conditions,the stemflow rate was the highest,reaching 9%.The stemflow lagged behind the start and end time of the rain.The delay of the start time and the end time and the extension of the duration of the stemflow were significantly affected by rainfall characteristics.The delay of the start time is significantly negatively correlated with rainfall intensity,and negatively correlated with the first-30min rainfall intensity.The delay of the ending time was only positively related to the rainfall amount.The prolongation of stemflow duration is significantly positively correlated with rainfall intensity.Among the tree morphology factors,the DBH diameter is significantly positively correlated with the stemflow.（4）Canopy interception accounted for 36%of the total rainfall in the study period,which was significantly related to the characteristics of rainfall.The interception rate is different under different rain types.Type I rainfall interception rate reached 52%,while Type IV rainfall interception rate was only27%.The measured canopy interception was 245.6mm.The simulation results of Gash model was 6.1%lower than the observed values,and the Nash-sutcliffe efficiency coefficient reached 0.74,indicating good performance of the model in simulating canopy interception progress of Populus×euramericana plantations.（5）The average thickness of litter under the canopy was 56.5mm,and the average actual water holding capacity is 14.18 t·hm^-2.The water absorption capacity was the highest in the first 5 minutes of the moisture absorption process,and the intensity of the drying process decreased with time.After rainfall,the average duration of soil water increasing to its maximum value was 628.3min under the canopy,while this duration lasted only 516min outside the forest.In the plantation,the replenishment time of soil water had a very significant positive correlation with the rainfall intensity in the first 30 minutes.The amount of soil water replenishment was significantly positively correlated with rainfall and rainfall intensity during the first 30 minutes.The RMSE reached between 1 and 2%and RMAE ranged within 2 and 13%for the Hydrus-1D simulation of the soil water movement in and outside the plantation.Moreover,NSE of the model was greater than 0,and reached higher than 95%in some soil layers,indicating a good performance of the model in simulating the soil water movement of Populus×euramericana plantation.According to the model simulation results,it was found that the soil moisture front movement was slower in the plantation than that outside the plantation,indicating that the soil’s ability to buffer rainfall was better in the plantation than that outside.Study showed that the canopy redistribution of rainfall by Populus×euramericana plantations can extend the throughfall duration,weaken the under-canopy rainfall intensity.Moreover,it extends the duration of the soil water replenishment process,improving the soil’s ability to receive recharge and buffer the rainfall erosivity of the soil.Our results have great significance for the assessment of the eco-hydrological function of Populus×euramericana plantations.