Modeling the Response of Daily Evapotranspiration and its Components of a Larch Plantation to the Variation of Weather, Soil Moisture, and Canopy Leaf Area Index

Abstract

An accurate prediction of forest evapotranspiration (ET) based on its components in response to a changing environment is essential for understanding interactions between the atmosphere, soil, and vegetation and for integrated forest‐water management. The ET components of a pure larch plantation and the environment were monitored over 2 years in northwest China. The response functions of each ET component to individual driving factors were determined using upper boundary lines, then coupled to form the ET component modules, fitted with measured data in 2016 (May–September), and validated with measured data in 2015 (June–September). Results showed that (1) the response of daily transpiration (T) to potential ET (ETref) followed a binomial equation, and the response of T to relative extractable water (REW) of the 0‐ to 60‐cm soil layer and canopy leaf area index (LAI) followed a saturated exponential growth function. The module was T = (−5.766 × 10−4ETref2 + 0.005ETref–0.002) × (18.769 + 46.990 (1–exp(−8.555REW))) × (−14.662 + 17.428 (1–exp(−1.414LAI))). (2) The response of daily forest floor evapotranspiration (FE) to ETref, volumetric soil moisture (VSM) of the 0‐ to 30‐cm soil layer, and LAI followed positive linear, saturated exponential growth and saturated exponential decay function. The module was FE = (6.697ETref–2.770) × (6.927–11.243exp(−1.959VSM)) × (0.032 + 1.162exp(−2.407LAI)). (3) The canopy interception (Ic) of individual rainfall events was mainly affected by precipitation amount (P) and LAI. The module was Ic = 0.446 LAI (1–exp(−0.112P)) + 0.097P. (4) The daily ET model was built up as ET = T + FE + Ic, which had good performance in both the calibration and validation period. It can be concluded that the variation of daily forest ET can be accurately predicted using the easily measurable driving factors of weather, soil, and vegetation.

The Variation in Water Consumption by Transpiration of Qinghai Spruce among Canopy Layers in the Qilian Mountains, Northwestern China

“…For Qinghai spruce trees, their sapwood depth normally was less than 40 mm and its sap flow density varied with sapwood depth by the Gaussian distribution function with a peak at the depth of 20 mm [29]. In this study, our probes were only 20 mm long, which could not measure the sap flow density within the sapwood deep exceeding 20 mm [13]. Thus, the J s in the 20 to 40 mm was calculated by Gaussian regression equation (Equation 5 To quantify the daily and monthly variation in transpiration among canopy layers, the daily transpiration (F i , mm•day −1 ) of each canopy layer per unit area was calculated by Equation 6:…”

“…Figure 6 shows the difference between the measured transpiration (T) and simulated transpiration (Tp), and a substantial negative deviation occurred during this period, indicating that transpiration was limited by soil moisture. When VSM was less than 0.21 m 3 •m −3 , the daily stand transpiration responded to VSM following a linear relation except on rainy days ( Figure 7); that is, the daily stand transpiration decreased with the decreasing VSM based on Equation (13).…”

“…The sap flow density (J s , mL•cm −2 •min −1 ) (flow per unit of sapwood area) of an individual sample tree was calculated by Equations (3) and (4) [4,13]:…”

“…According to the Food and Agriculture Organization of the United Nations (FAO) Penman-Monteith equation, the daily potential evapotranspiration (PET, mm•day −1 ), as a comprehensive meteorological index of many weather parameters (air temperature, solar radiation, VPD, and wind speed), can reflect the evaporative demand determined by the air conditions and available energy and thus has an important effect on transpiration [13]. The daily PET was used to estimate by using the Equation (9) [39]:…”

“…Transpiration is strongly dependent on potential evapotranspiration (PET) and volumetric soil moisture (VSM) [4,13,16,43], especially in arid and semiarid areas where the soil moisture is rather low and the evaporative demand is high [4,6].…”