| To solve the problems of water resources decreasing from the Yellow River,the renovation project of water-saving and water distribution controlling had been implemented in the Hetao Irrigation District of North China since 1999.The total water diversion from Yellow River will be reduced from 5.2 billion m3 in 2000 to 4 billion m3 in 2015.The local hydrological cycle would be inevitably influenced or changed.During the past fifteen years,the crop pattern and the groundwater level of the irrigation area have changed greatly,and these changes will have a direct impact on farmland evapotranspiration and changes in soil and water environment.In this paper,the major crop types were extracted successfully based on the Spectral matching technique(SMT)and data fuison method from 2000 to 2015.The SEBS(Surface Energy Balance System)model and the ESTARFM(Enhanced Spatial and Temporal Adaptive Reflectance Fusion Model)algorithm were applied to produce daily field-scale evapotranspiration(ET)based on Landsat and MODIS data from 2000 to 2015 at intervals.The mian works are to reconstruct the daily field-scale ET using Landsat-ET and MODIS-ET datasets and evaluate the performance of the ESTARFM;to analyze the seasonal and interannual variation of ET for different crops and the controlling factors on ET in the study area;to analyze the temporal and spatial characteristics of agriculture ET and its response to crop pattern and groundwater depth;and to evaluate the impacts of water-saving renovation project.Surface and root zone soil storage are the most direct indicators to reflect the agricultrue drought.The surface and root zone soil moisture are constructed based on the LST-NDVI index.And then the data fusion method was been used to obtain field-scale soil storage with different crop types,aiming to provide guidance for agricultural irrigation and agricultural production.The results showed:(1)The crop pattern was changed greatly from 2000 to 2015,which influenced mainly by the agricultural products market and national economic policies.Both the maize and wheat planting area increased,especially the maize acreage increasing from 0.83×104 hm2 to 4.02×104 hm2.The sunflower acreage decreased before 2005 and then increased gradually.Due to the outflow of rural labor and high labor costs,the interplanting area was reduced from 4.30×104 hm2 to 0.41×104 hm2.For spatial distribution,the maize and wheat are more concentrated in the southeastern,north and northeast part of the irrigation region,and sunflower is distributed in the western and noreheastern of the region.The interplanting patterns are mainly concentrated in the southeastern and southwest.(2)The process of fusion ET was more consistent with ET output from water balance.In the correlation analysis of water balance and fusion ET,the determination coefficient R2 of maize,wheat and sunflower reached 0.85,0.79 and 0.82 respectively.During the growth period,the root mean square errors(RMSE)of maize,wheat and sunflower were lower than 0.70 mm/d,the mean absolute error(MAD)was all lower than 0.75 mm/d,and the relative error(RE)was all less than 16%.On the spatial scale,the spatial characteristics of fusion results were consistent with the Landsat ET.The correlation coefficients of the July 23,August 24 and September 1,respectively,reached 0.85,0.81 and 0.77,the mean values of the differences were 0.24 mm,0.19 mm and 0.22 mm,respectively.And the standard deviations were 0.81 mm,0.72 mm and 0.61 mm.The high resolution ET based on ESTARFM fusion algorithm is reliable and has good fusion precision.(3)The evapotranspiration of different crops varied greatly both in the growth period and non-growth period.During the growth period,the maximum of water consumption was interplanting for 637 mm,followed by maize and sunflower,respectively for 598 mm and 502 mm,the minimum water consumption of wheat was 412 mm.During the non-growth period,wheat had the highest water consumption with an annual average of 214 mm,maize and sunflower were 42 mm and 128 mm,respectively.The spatial distribution of agricultural ET has not been changed obviously along with the great adjustment of crop pattern,while its spatial characteristics was in good accordance with the groundwater depth with the higher values occurred in shallow groundwater areas and lower values appeared in deep groundwater areas.The total agricultural ET still kept in a stable level and was not obviously sensitive to the adjustment of crop pattern and decline of groundwater level.(4)Since the water-saving renovation program and the reduction of diversion from Yellow River,the groundwater depth descended from 1.76 to 2.33.This would reduce the invalid evaporation and alleviate the soil secondary salinization effectively.Meanwhile,the net irrigation water use efficiency was improved,which indicated powerfully the positive impacts of water-saving project on the local water hydrological environment.However,Kc,a decreased with an average annual decline of 0.3%during the study period.It is noteworthy that the continuous reduction of net irrigation water and decline of water table would lead to the possible drought stress.(5)The soil water moisture can be well evaluated with LST-NDVI index both at Landsat and MODIS spatial scale.The simulated surface soil water content have a good correlation with the observation with the determination coefficient of 0.743 for Landsat and 0.569 for MODIS.Similarly,the simulated root zone soil water content also agreed well with the observation with the determination coefficient of 0.772 for Landsat and 0.585 for MODIS.In the non-irrigated period,the field-scale soil moisture can be effectively evaluated by data fusion.The results are close to the observed value.The relative error of wheat is less than 20%and the relative error of maize and sunflower are less than 10%.During the irrigation period,due to the irrigation events,the remote sensing image did not capture the great change of the surface soil moisture,the error between the result and observation was larger.The maximum error of wheat appeared on May 15 with 58.48%,and sunflower maximum error occurred in July 10 and July 13,respectively,with 12.91%and 33.56%.The fusion result in irrigation period is not as good as non-irrigation period.(6)In the most area of fields of maize,sunflower and wheat,the value of relative humidity was more than 60%and the area in drought was less.The proportion of light drought area of maize on June 18,July 28,August 27 and September 28 were 8.95%,5.46%and 3.16%and 15.91%respectively.The proportion of light drought area of sunflower on June 18,July 28,August 27 and September 28 were 7.04%,5.22%,3.64%and 6.19%respectively.For the wheat,the proportion of light drought accounted for 8.91%and 13.49%.During the whole growing season,there were no heavy drought appearing in the fields of maize,sunflower and wheat. |
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