Evaluating methods of estimating global radiation and vapor pressure deficit using a dense network of automatic weather stations in coastal Brazil.

Abstract

A dense network of automatic weather stations (AWS) in eastern Brazil provided the opportunity to test established relationships between global radiation (Rs), photosynthetically active radiation (Rp) and net radiation (Rn). We also examined the variation in vapor pressure deficit (VPD) across the region, and predictions of VPD from temperature data. Predictions of Rs using the MTCLIM package [Agric. For. Meteorol. 93 (1999) 211] accounted for 75% of the variance (r2 = 0.75) in monthly mean measured values but only 62% of the variance in daily values. A procedure described by [Int. J. Bio-meteorol. 44 (2000) 204] was less accurate. Relationships between Rn and Rs gave lower intercept values (indicative of net long-wave fluxes) than expected. Data for a year gave a value of 0.43 for the ratio of Rp to Rs; instrumental problems prevented longer-term comparisons. VPD during daylight hours (VPDday) varied significantly between the northernmost weather station (at 17°26′S) and the most inland, at 17°55′S but at slightly higher altitude (66m compared to 160 m). The r2 values for the linear relationships between maximum and minimum temperatures and VPD day varied across the region, ranging from 0.52 to 0.79. Using a process-based forest production model (3-PG; [For. Ecol. Manage. 95 (1997) 209]) we show that differences in VPD can lead to considerable (28%) reductions in the yield of plantation eucalyptus.