Ecohydrological Processes In A Large Irrigated Area Of The North China Plain: Field Observation And Modeling

The North China Plain is one of the main crop-producing regions in China.However, limited water resources and large amount of irrigation water use have led toserious water shortage. As the climate change is being much more concerned,quantitative investigation of the water and carbon cycles is essential for efficient watermanagement and CO2sequestration in this region. The coupled water-energy-carbonflux exchanges between land and atmosphere are the dominant ecohydrologicalprocesses in agroecosystems of this region. Therefore, this thesis focused on thevariability of these dominant ecohydrological processes though a long-term fluxobservation, and then attempted to develop an ecohydrological model for exploringvariations in the ecohydrological processes in the past and projecting changes of theseecohydroogical processes in the future.The Weishan Irrigation District, a typical irrigated area in the North China Plain,was selected as the study area. A flux tower was set up in a typical cropland in thisirrigated area, aiming to observe the water, energy, and carbon fluxes and otherassociated hydro-meteorological elements. The inter-comparison of fluxes observed bydifferent instruments showed that the accuracy of the flux observations was acceptablyhigh. Analysis of the observed data showed that, the interannual variability ofevapotranspiration (ET) was controlled by the equilibrium evaporation, while theintra-annual variability of ET was controlled by both of the equilibrium evaporation andthe canopy conductance. During the peak growth seasons of winter wheat and summermaize, the ratio of latent heat flux to net radiation was83%and57%, respectively.Annual average value of this ratio was59%. In seasonality, the temperature response ofecosystem respiration (R_eco) and the light response of net ecosystem exchange (NEE)corresponded closely to the crop phenology. The seasonal variations in R_ecowereprimarily controlled by both of the gross primary production (GPP) and air temperature,while the interannual variations in R_ecoand GPP were primarily controlled by airtemperature. Annually, this cropland was a strong carbon sink, but could be possiblyconverted into a weak carbon source when the grain harvest was taken into account.A Hydrologically-Enhanced Land Process model (HELP) was developed based onthe Simple Biosphere Model2(SiB2). A field-scale ecohydrological model (HELP coupled with Crop growth model, HELP-C) was then developed through coupling theHELP with a crop growth model, which realized the modeling of crop growth.Validations by the observed data showed that the HELP and HELP-C were effectivetools for simulating water, energy, and carbon fluxes. Finally, a regional-scaleecohydrological model was developed through coupling the HELP (or HELP-C) with adistributed hydrological model. This regional-scale ecohydrological model can be usedfor simulating the historical variations in water and carbon cycles, based on remotelysensed vegetation index. Optionally, this model can be used for projecting the possiblechanges of water and carbon cycles based on the future climate scenarios.The model simulations during1984²006showed that the interannual variabilityof ET was small due to the sufficient irrigation even though the interannual variabilityof precipitation was high. Likewise, the annual ET had no significant long-term trend.The accumulated ET in the wheat and maize seasons were284and314mm,respectively, corresponding to99%and71%of the total amount of precipitation andirrigation, respectively. The annual net primary production (NPP), soil respiration (Rs),and NEE showed no significant long-term trend. However, the net biome productionshowed significant increasing trend, and the accumulated NPP in the wheat and maizeseasons showed significant increasing and decreasing trends, respectively. Projectionunder the future climate scenarios (2010²049) and the sufficient irrigation conditionshowed that the ET in the wheat and maize seasons would decrease by10%and7%,respectively. However, the irrigation water requirement of wheat and maize wouldchange by+1%and-22%, respectively. The NPP in the wheat and maize seasons wouldincrease by11%and3%, respectively, but Rsin both crop seasons would change veryslightly. The wheat and maize yields would change by+21%and-3%, respectively.