## 2 Study Area and Observations  
The headwater QMs of the HRB (97\({}^{\circ}\)24\({}^{\prime}\)E-102\({}^{\circ}\)10\({}^{\prime}\)E and 37\({}^{\circ}\)41\({}^{\prime}\)N-42\({}^{\circ}\)42\({}^{\prime}\)N) were selected as the study area because it is ecologically and hydrologically important for agricultural irrigation within the middle reaches (Hexi Corridor) and maintains ecological viability within the lower reaches (northern Alxa Highland). Geographically, the landscapes are diverse and include glaciers, frozen soils, alpine meadows, and the forests of the QMs; irrigated crops within the Hexi Corridor; and riparian ecosystems, deserts, and Gobi within the Alax Highlands (Figure 1). The land use map in Figure 1 was first generated using TM data acquired in 2009 [53]. The map was upscaled to 1 km in order to maintain the same resolution as the driving data (_i.e._, the MODIS products and the downscaled WRF estimates) for the ET model. Spanning an area of 10,400 km\({}^{2}\) and with altitudes from 1500 to 6000 m above sea level, the vegetation types of the QMs vary from high to low altitudes in the following sequence: alpine meadows and shrubbery, forests, and drying shrubbery and desert steppes. Forests in the area are largely composed of _Picea crassifolia_ mixed with a fairly small fraction of _Sabina prezewalsk_ that only survives on shady slopes (from altitudes of 2500-3300 m) and that occupies 2.97% of the total area. Lying along the northeastern margin of the Tibetan Plateau, the QMs have a temperate continental mountainous climate with a warm and humid summer and a cold and dry winter. Mean annual precipitation and mean annual temperature display a decreasing trend from the southeast to the northwest. Mean annual precipitation (with approximately 90% occurring during May and October) decreases from approximately 600 mm to less than 200 mm. The mean annual air temperature is \(\sim\)6 degrees (Celsius) at low altitudes and \(\sim\)\(-\)10 degrees (Celsius) at high altitudes. To improve the observability, understanding, and predictability of eco-hydrological processes on the catchment scale, the Water Allied Telemetry Experimental Research (WATER) campaign that spanned from 2007 to 2011 conducted multi-scale and simultaneous airborne, space-borne, and ground-based remotely sensed experiments within the HRB [54, 55]. A network of automatic weather stations (AWS), surface meteorological towers, and EC stations were established by WATER. Only observations from the mountainous forest site (Guantan, 100\({}^{\circ}\)15\({}^{\circ}\)E, 38\({}^{\circ}\)32\({}^{\circ}\)N, 2835 m) were used in this study. The forest EC system contained a three-dimensional sonic anemometer (CSAT-3, Campbell, Inc., Logan, UT, USA), a CO\({}_{2}\) and H\({}_{2}\)O gas analyzer (LI-7500, LI-COR, Inc., Lincoln, NE, USA), a heat flux plate (HFP01, Campbell, Inc., USA), a four-component radiometer (CM3 and CG3, Campbell, Inc., USA), a temperature and relative humidity probe (HMP45C, Vaisala, Inc., Helsinki, Finland), a wind speed sensor (014A and 034B, Met One Instruments, Inc., Grants Pass, OR, USA), and a data logger (CR5000, Campbell, Inc., USA). Data quality control processes were applied to the raw 10 Hz EC data in order to obtain half-hourly flux data [56]. Processing steps included despiking, coordinate rotation, a time lag correction, a frequency response correction, a WPL correction, and gap filling [56, 57, 58]. The gap-filling process was based on two methods: (1) a Look Up Table (LUT) when only EC data were missing, and (2) Mean Diumal Variations (MDV) when both EC and AWS data were missing. Due to low data quality and data gaps, measurements obtained from 2008-2009 were used for this study. Measurements obtained from 2010-2011 at the mountainous forest site were used to compare ETs obtained from the SEBS. Two forest inventory surveys were conducted from June to August during 2007 and 2008.