The wetness of high-latitude land surfaces is strongly dependent on the dif- ference between precipitation (P) and evapotranspiration (ET). If climate models are to capture the trajectory of surface wetness in high latitudes, they must be able to simulate the seasonality and variations of the surface moisture fluxes, as well as the sensitivities to the variations to the drivers. In this study, a combination of regional climate model output and eddy covariance mea- surements from flux tower locations in Alaska is used to evaluate model si- mulations of the surface moisture fluxes and their variations. In particular, we use the model output and the field measurements to test the hypothesis that temperature (T) is the key driver of variations of ET in tundra regions under- lain by permafrost, while precipitation plays a greater role in boreal forest areas. Although the model’s hydrologic cycle is stronger (larger P, larger ET) relative to the in situ measurements at all the sites, the prominent seasonal cycles of P, T, and ET are captured by the model. The tower measurements from all sites show a short period (one or two months) of negative P-ET dur- ing summer, indicative of surface drying, although the model does not show this period of drying at the inland tundra site. At all the tundra sites, both the flux tower data and the model simulations show that daily and warm-season totals of ET are largely temperature-driven. Daily ET shows a weak negative correlation with precipitation in the measurements and in the model results for all the sites. Precipitation is the main driver of year-to-year variations of the seasonally integrated net moisture flux at all the sites, implying that preci- pitation will be at least as important as temperature in the future trajectory of surface wetness.
