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A combination of measurement and modelling was used to find a pragmaticsolution to estimate the annual total evaporation from the rare andindigenous Nkazana Peat Swamp Forest (PSF) on the east coast of SouthernAfrica to improve the water balance estimates within the area. Actual totalevaporation (ETa) was measured during three window periods (between7 and 9 days each) using an eddy covariance (EC) system on atelescopic mast above the forest canopy. Sap flows of an understory tree andan emergent tree were measured using a low-maintenance heat pulse velocitysystem for an entire hydrological year (October 2009 to September 2010). An empiricalmodel was derived, describing the relationship between ETa from theNkazana PSF and sap-flow measurements. These overlapped during two of thewindow periods (R2 = 0.92 and 0.90), providing hourly estimates ofETa from the Nkazana PSF for a year, totalling 1125 mm (while rainfallwas 650 mm). In building the empirical model, it was found that to includethe understory tree sap flow provided no benefit to the model performance. Inaddition, the relationship between the emergent tree sap flow with ETabetween the two field campaigns was consistent and could be represented by asingle empirical model (R2 = 0.90; RMSE = 0.08 mm h−1). During the window periods of EC measurement, no single meteorologicalvariable was found to describe the Nkazana PSF ETa satisfactorily.However, in terms of evaporation models, the hourly FAO Penman–Monteithreference evaporation (ETo) best described ETa duringthe August 2009 (R2 = 0.75), November 2009 (R2 = 0.85) andMarch 2010 (R2 = 0.76) field campaigns, compared to thePriestley–Taylor potential evaporation (ETp) model(R2 = 0.54, 0.74 and 0.62 during the respective field campaigns).From the extended record of ETa (derived in this study fromsap flow) and ETo, a monthly crop factor (Kc) wasderived for the Nkazana PSF, providing a method of estimating long-term swampforest water-use from meteorological data. The monthly Kcindicated two distinct periods. From February to May, it was between 1.2 and1.4 compared with June to January, when the crop factor was 0.8 to 1.0. Thederived monthly Kc values were verified as accurate (to onesignificant digit) using historical data measured at the same site, alsousing EC, from a previous study. The measurements provided insights into the microclimate within a subtropicalpeat swamp forest and the contrasting sap flow of emergent and understorytrees. They showed that expensive, high-maintenance equipment can be usedduring manageable window periods in conjunction with low-maintenance systems,dedicated to individual trees, to derive a model to estimate long-termETa over remote heterogeneous forests. In addition, the contrastin annual ETa and rainfall emphasised the reliance of the NkazanaPSF on groundwater

hydrology and earth system sciences

Extending periodic eddy covariance latent heat fluxes through tree sap-flow measurements to estimate long-term total evaporation in a peat swamp forest