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

A dense network of automatic weather stations (AWS) in eastern Brazil provided the opportunity to test established relationships between global radiation (R_s), photosynthetically active radiation (R_p) and net radiation (R_n). We also examined the variation in vapor pressure deficit (VPD) across the region, and predictions of VPD from temperature data. Predictions of R_s 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 R_n and R_s 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 R_p to R_s; instrumental problems prevented longer-term comparisons. VPD during daylight hours (VPD_day) 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.