| Groundwater evapotranspiration(ET_G)is one of the vital factors of groundwater resource management and environment protection in arid and semi-arid areas,as the water resources are scarce and the ecological environment is fragile.The method of diurnal water table fluctuations is widely used to estimate ET_G in groundwater dependent ecosystems.In this paper,the recharge process of groundwater to capillary water at night was proposed for the first time,according to the inducing mechanism of diurnal water table fluctuations.While,the estimation method of daily water recovery in capillary zone was explored,and then the method of water table fluctuations was modified.Secondly,the migration mechanism of groundwater at night was revealed,and the mechanism of underestimating ET_G by traditional methods was clarified using the HYDRUS-1D model.Combined with the measured data of arid area in NW China,the calculation accuracy of the modified method was verified effectively.Then,an automated observation system was established in the Mukainao watershed of Ordos Plateau in the arid region of NW China,which realized multi-element and multi-scale monitoring of groundwater,soil and meteorology,and revealed the temporal and spatial variation of ET_G in the study area.Finally,the sensitivity of ET_Gto meteorological conditions and depth to water table(DWT)in different phreatophyte areas was found out.The main conclusions are as follows,(1)The model study showed that groundwater can replenish the water deficit of capillary zone at night under different conditions.The daily capillary water recovery is affected by the DWT and soil textures.Compared with the traditional method,the average calculation accuracy of the modified method is improved by 52%.In addition,the field measured data showed that,compared with the traditional method,the calculation results of the modified method at the daily and hourly scales had a better correlation with the reference crops evapotranspiration.The verification results of both model data and field measured data showed that the modified method is much more accurate than the traditional method in the calculation of ET_G.(2)The daily capillary water recovery exists in all observation wells in the study area,which is mainly affected by vegetation type.The temporal and spatial variation of ET_G showed that solar radiation was the main influencing factor on the hourly scale;relative humidity was the main influencing factor on daily scale;and the main influencing factor of monthly scale was air temperature.Vegetation type and DWT were the main factors influencing the spatial variation of ET_G.The spatial difference of ET_G reflected that the ET_G calculated based on the diurnal water table fluctuations of a single observation well can only represent the point-scale evapotranspiration,rather than the mean level of ET_G in large-scale regions.(3)In the vegetation growing season,the average ET_G of Achnatherum splendens,Carex stenophylla,Salix psammophila and Populus alba was 4.10 mm/day,5.87 mm/day,8.96 mm/day and 9.89 mm/day,respectively.ET_G in the forest area was about twice that in the grassland area.Additionally,the greater the DWT in the vegetation area,the less the influence of meteorological factors.After eliminating the influence of meteorological factors,the normalized ET_G decreased exponentially with the DWT.According to the attenuation coefficient of the exponential equation,the sensitive degree of different vegetation to water table change was Populus alba,Carex stenophylla,Achnatherum splendens and Salix psammophila in descending order. |
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