Evapotranspiration Of Haloxylon Ammodendron Plantation In Minqin Oasis–desert Ectone

The oasis-desert ecotone is the main place for element cycles, energy exchange and information transmitting between oasis ecosystems and desert ecosystems, which plays a critical roles in maintaining ecological safety of the oasis ecosystems as well as the development of economics and survival of local people. Evapotranspiration, one of the most important processes in both the hydrological and ecological element cycles in the oasis-desert ecosystems, is closely related to the energy exchange in the system of soil-plant-atmosphere continuum(SPAC), contributing to maintain the water-heat balance of SPAC. Minqin area, located at the lower reaches of Shiyang river between Tengger and Badanjaran desertsin Gansu Province, is suffering serious vegetation degradation and land desertification due to climate change and heavy human disturbances. Establishing of Haloxylon ammodendron plantation on degraded land is the most successful way to prevent land desertification in the Minqin basin. Therefore, study on the evapotranspiration and transpiration characteristics of H. ammodendron plantation will provide scientific evidences that may be great helpful to understand the eco-hydrological processes and the policy-makers in the northwest China.In the present study, systematic filed observations for H. ammodendron plantation were conducted at the Gansu Minqin Desert Control Experiment Station, including the evapotranspiration, transpiration, and associated environmental factors, which were explored using the eddy covariance system(EC), the thermal dissipation sap flow velocity probes(TDPs) and the time domain reflectometer(TDR) etc. during June, 2014 to December, 2015. The main results were showed as following:(1) The average of energy balance ratio(EBR) of H. ammodendron plantation was 0.88, indicating that the data quality using the instruments of eddy covariance system were acceptable and precise for the H. ammodendron plantation in Minqin oasis-desert ectone.(2) The energy balance components showed clearly seasonal variation from 2014 to 2015. The sensible heat flux(H) was the major daily energy expenditure, while the latent heat flux(LE) was the second expenditure because of the weather condition of dry-hot, strong radiation. The variations of energy components ratio was different obviously at daily and seasonal time scales. The ratio LE to net radiation(Rn) was only 21.19%. The ratio H to Rn was 62.31%. The energy partitioning was influenced greatly by precipitation. For example, LE sharply increased after precipitation in the first day, and then decreased on the fourth day. After slowly descending of H after precipitation, H slowly increasedat the third or fourth day after precipitation, which the soil heat flux decreased slightly.(3) During 2014 to 2015 year, the actual ET amount was 191.31 mm with the precipitation(P) amount of 199.9 mm, and the ratio ET to P was 0.96 in the H. ammodendron plantation The soil moisture was the major factor to control ET, and ET was mostlydriven by net radiation(Rn). The conclusion that the water budget approximated balance. The ET process was significantly influenced by climate factors and soil water, especially rainfall in the plantation. The H. ammodendron ecosystem is fragile.(4) The diurnal variation of stem sap flow(Js) showed evidently diurnal variations. The annual water consumption was 126.74 mm and 166.01 mm in 2014 and 2015, respectively. The ratio of total transpiration and precipitation was 1.38 in 2014 and, 1.36 in 2015.The air temperature was the main microclimate factors to drive the transpiration of H. ammodendron. After a strong rainfall, ET can sharply increased in the first day following with strong soil evaporation, indicating the weak water retention ability in the H. ammodendron plantation. The degree of correlation between transpiration and soil water in root area was similar to that between transpiration and air temperature, vapor pressure deficit and net radiation.