Abstract:Sap flow is a key character in the understanding of plant hydraulic functioning and it is crucial in determining water consumption research of agriculture in Northwest China. The field experiment was conducted during 2008—2010 at the Shiyanghe Experimental Station for Water-Saving in Agriculture and Ecology. The site has a continental temperate climate with mean annual sunshine duration of more than 3 000 h, mean annual temperature of 8℃ and mean annual precipitation of 164 mm. Apple trees ( Malusdomestica Borkh . cv. Golden Delicious) were planted in 1981, with row spacing of 6 m and plant spacing of 4 m. The experimental plots were irrigated four times per year by flood irrigation to avoid tree water stress. After measuring trunk sap flow using heat pulse method and wind speed, net radiation, relative humidity, air temperature with an automatic meteorological station, the seasonal variation of sap flow was analyzed and the daily transpiration was modeled using seasonal evapotranspiration model by canopy gap distribution (Cj), dual crop coefficient method ( K cb ) and Priestley-Taylor method (PT). The Cj model combined hourly changes in canopy gap fraction to calculate the cover ratio of soil under the canopy and bare soil outside the canopy for soil evaporation, which was constant in the clumping model. Results showed that the seasonal sap flow increased at the beginning of the growing stages and reached peak value of 6.5 mm/d in mid-June. The Cj model performed well in modeling daily transpiration. Compared with the trunk sap flow, the average transpiration, determination coefficient ( R 2 ), mean absolute error (MAE) and Nash and Sutcliffe modeling efficiency (NSE) of Cj model were 2.51 mm/d, 0.49, 0.93 mm/d and 0.01, respectively. The Cj model and PT model were better in the leaf expanding period than in fruit expanding period for transpiration modeling, while the K cb model kept stable during the whole season. The Cj model could decrease the effect of time lag of sap flow and be used for long time daily transpiration modeling. The long time scale transpiration modeling provided an efficient method for establishing the irrigation schedule in Northwest China.
