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This paper summarizes the initial results (first 4 yr) of a whole‐catchment manipulation to evaluate the impact of a loss of hydrologic connectivity of a boreal lake with its watershed on lake thermal and evaporative regimes. We diverted the upstream flow to a fourth order lake resulting in a marked reduction (81%) in watershed area, and compared the response to a comparable control lake in a headwater position. The manipulation reduced runoff into the experimental lake from 2400 mm to 90 mm, greatly increased theoretical residence time (from 2.3 yr to 18.3 yr), and reduced average dissolved organic carbon concentration from 5.8 ± 0.3 mg L−1 to 5.5 ± 0.3 mg L−1. Average Secchi depth was increased by 0.6 m, resulting in a 0.25 m deeper epilimnion and cooler surface water temperatures (8% of the time) than those predicted by pre‐manipulation relationships. There was some evidence that the response of evaporation to episodic events was altered, but this was not extensive enough to alter annual evaporation or median evaporation rates. Our findings show that the impacts of hydrologic connectivity on lake chemistry can cascade to alter the energy budgets of boreal lakes. These results can inform how differences in hydrological connectivity across the landscape due to lake‐watershed topology and climate will impact boreal lakes. Of note is that very clear lakes in the boreal region are highly sensitive to impacts from a change in clarity. Information on their spatial distribution will be necessary to assess impacts of hydrological connectivity on the boreal lake complex. [ABSTRACT FROM AUTHOR]

Summary: Evaporation is an important part of the water balance of a catchment, and estimates of potential evaporation (PE) are an important input to hydrological models. When modelling the hydrological impacts of climate change, using data from climate models, present models generally do not provide direct estimates of PE from the land surface. Estimates thus have to be calculated from other climate variables, and many existing formulae can be applied. This paper compares the well-established, more physically based but data-intensive, Penman–Monteith PE against a simple, temperature-based (T-based) PE, when calculated from readily available monthly climate model data over Britain (for five global and eight regional climate models). The performance of the two PE formulations is compared to MORECS PE (a gridded dataset derived by the UK Met Office from weather observations, using a modified Penman–Monteith formulation) for the baseline period 1961–1990, and the changes in the two PE estimates between the 1970s and the 2080s are compared. The results show that the T-based PE matches MORECS PE better than does Penman–Monteith PE, for all the climate models studied. However, the changes in the two types of PE between the 1970s and 2080s are different, for each of the climate models, and these affect the modelled hydrological impacts. This is illustrated using three example catchments spread across Britain. The uncertainty introduced by the PE formulation is less than that due to the climate model, but could still be important for some applications. [Copyright &y& Elsevier]