Background: Evapotranspiration (ET) is an important process of ecosystem water cycle, including soil evaporation (E) and plant transpiration (T). Distinguishing the two parts is helpful to understand the water vapor exchange process and the contribution of E/T to ET in the water dissipation structure of Soil—Plant—Atmosphere Continuum (SPAC). Larix gmelinii is the main constructive species of forest ecosystem in cold temperate zone, the diurnal variation of oxygen isotope composition (δ18O) and its quantitative differentiation in ET components in main Larix gmelinii forest types were studied. It can provide scientific basis for regional water resources protection and forest ecosystem management. Methods: The Off—Axis Integrated Cavity Output Spectroscopy (OA—ICOS) was used to measure the water vapor concentration and its δ18O in 6 vertical gradients of the Larix gmelinii ecosystem. During the vigorous growth period of plants (June to July), plant branches and soil samples of 5 main forest types in the ecosystem were collected on typical sunny days. The δ18O values of the samples were determined by condensation vacuum extraction and liquid water isotope analyzer. Then, the Craig—Gordon model was used to estimate the δ18O of E, the δ18O of T is estimated based on the Isotopic Steady State (ISS) assumption, the δ18O of ET was estimated by Keeling plot equation. Finally, the binary linear mixed model was used to estimate the contribution rate of ET component. Results: The results showed that: (1) the variation trend of water vapor concentration and its δ18O in different vertical gradients of Larix gmelinii ecosystem showed a "V" diurnal variation trend, with the minimum values appeared between 11:00 and 17:00, and the δ18O decreased with the increase of height. The δ18O of ET showed a “inverted V” diurnal variation trend, with the maximum values between 10:00 and 14:00, which was opposite to the variation trend of the ecosystem water vapor δ18O; (2) the δ18O of E in different forest types increased in daily scale, ranging from －35.71 ‰ to －29.12 ‰, which was smaller than the δ18O of soil surface water. The δ18O of T in different forest types decreased in a daily scale, ranging from －14.22‰ to －11.21‰, which was smaller than the δ18O of plant water; (3) the contribution of T to ET was the largest from 10:00 to 14:00, with an average of 79%—96%. The order of T contribution rate for each forest type from large to small was BL (75.49 ± 2.06%), RL (74.72 ± 2.12%), PL (72.62 ± 2.01%), SL (72.42 ± 2.78%) and LL (72.35 ± 1.99%). The contribution rate of E was the highest from 06:00 to 10:00, with an average of 28—47%. Conclusions: In the vigorous growth period of the Larix gmelinii ecosystem, the diurnal variation of δ18O and its quantitative differentiation in ET components of different forest types were significant. T was the main source of the ET, on the whole, the contribution rate of T in shrub dominated Larix gmelinii forest was higher than that in grass dominated forest type , and moss dominated forest type.

Based on observed precipitation and runoff data, monthly actual evapotranspiration (ETa) was calculated by the hydrological budget balance method in the Nu River Basin (NRB) and Lancang River Basin (LCRB). The performance of three developed complementary relationship methods, the nonlinear advection-aridity (nonlinear AA) method, generalized complementary relationship method (B2015), and sigmoid generalized complementary function (H2018), on simulating (ETa) were evaluated. The evaluation results showed that three methods were able to accurately simulate monthly (ETa) series. The NSE between the monthly (ETa) simulated by the nonlinear AA, B2015, and H2018 methods and the water-balance-derived (ETa) were 0.89, 0.83, and 0.91, respectively. The R-square were 0.90, 0.84, and 0.93, respectively. Overall, the H2018 method showed the best performance. The parameter α had a negative correlation with regional aridity index. Annual (ETa) and precipitation showed significant increasing trends during 1956–2018 in the basins at all temporal scales (dry and wet seasons and annual series). Runoff also exhibited an increasing trend in each sub-basin, except for the downstream region of the LCRB. The increasing magnitudes of wet reason precipitation and runoff in the mid-stream region was the highest, with the value of 73.7 mm/10a and 44.9 mm/10a, respectively. The (ETa) increased dramatically in the downstream region, the magnitude reached 25.9 mm/10a. Precipitation was the main factor leasing to (ETa) change. The increasing magnitude of (ETa) accounted for 42.4% of the precipitation increment. Research on the influence mechanism between meteorological factors and (ETa) showed that the contribution rate of air temperature to (ETa) was the highest, reaching 23.5%, which showed a significant positive correlation. The second was wind speed, whose contribution rate was − 10.2% on average, and even reached − 14.1% in the upstream region of the NRB. The correlation coefficient between (ETa) and wind speed was highest in mid-stream region of the NRB, which was greater than 0.80. The contribution rates of increasing humidity to (ETa) were − 12.5% and − 9.2% in the NRB and LCRB, respectively. (ETa) was negatively correlated with humidity. The negative correlation was especially strong in the mid-stream region, with coefficients were greater than − 0.65. The sunshine hours had the least effect on (ETa), and the contribution rates were − 6.5% and − 4.1%, respectively.