The impact of site factors and climate variability on the calculation of potential evaporation at Moel Cynnedd, Plynlimon S. B. Crane and J. A. Hudson Institute of Hydrology (Plynlimon), Staylittle, Llanbrynmair, Powys, SY19 7DB, UK.
Abstract. The meteorological record from the manual Moel Cynnedd
climate station at Plynlimon in the Welsh Uplands has been supplemented with solar radiation data, initially
from the Institute of Hydrology's Dolydd Office, and later from an adjacent automatic weather
station, in order to calculate Penman potential evapotranspiration for the entire 27 year
data set, 1969-1995. The methods of data capture are consistent with Meteorological Office criteria
throughout the entire record, establishing an unbiased and probably unique indicator of climatic
variability and change for this type of environment. Values of Penman Et calculated from these
data provides an independent index of atmospheric demand for moisture as an adjunct
to the hydrological studies being carried out in the Plynlimon and neighbouring catchments. Analysis
of the long term data indicates considerable year-to year variability in the component variables,
including some cyclical changes and possible long term trends in measured temperature. Annual
variability in Et is less than in the component variables, and there is an indication of a possible
long term cycle, but no evidence of an overall trend in Et during this particular study period.
The results indicate that some of the observed variability can be explained by inevitable changes
in exposure within this forest clearing site rather than changes in regional or global climatic
patterns. A single meteorological station sited in a forest clearing at a relatively low
altitude may underestimate potential evaporation across the catchment, as this will also include
areas of exposed hillside and forest canopy.
Citation: Crane, S. B. and Hudson, J. A.: The impact of site factors and climate variability on the calculation of potential evaporation at Moel Cynnedd, Plynlimon, Hydrol. Earth Syst. Sci., 1, 429-445, doi:10.5194/hess-1-429-1997, 1997.