poster - International Conference of Agricultural Engineering

2
However, the cultures are not always installed on horizontal surfaces, there are variations in
both the slope as the exposure field as a function of the solar declination. This fact may lead
to significant errors in the estimates of evaporation, in order that differences in the radiation
quantities received by various slopes can be quite high. Therefore, the study of the
relationship between the radiation incident on horizontal and inclined surfaces is desirable to
be able to minimize errors in the estimates of evapotranspiration through formulas that use
this variable as input, with the objective of rationalizing water in agriculture without affecting
productivity. To determine the radiation balance, when it does not have the necessary
sensors, it is used the formula recommended by FAO which depends on the measurement of
various parameters and coefficients that may not be suitable for the study area, occurring
significant errors in estimated values. For this reason several researches (André & Volpe,
1988; Marin et al., 2001; Alados et al. 2003; Silva et al. 2007) has sought to correlate the
radiation balance with the global solar radiation, thus obtaining regression equations with only
one input variable. Considering that the amount of incident solar radiation varies with the
exposure and the slope of vegetated surface plots (Chang, 1968), with water in the soil
sufficient to maintain the vegetation in conditions of maximum evaporation, logically will have
different evaporation rates at different slopes for a single exposure or in different exposures
to the same slope or when there is a combination of exhibitions and different slopes, as can
occur normally in a watershed and often in an agricultural property. Studies were not found in
the literature that analyzes comparatively the effect of exposure and the slope of vegetated
surfaces on the balance of solar radiation. This fact seems to be connected to the great
difficulty to be available, under natural conditions in the same place and with same type of
soil, areas with equal slopes and varied exhibition and vice versa, that studies with the aim of
determining the solar radiation balance in these situations can be realized. Research that
makes possible this amount of information could only be developed on surfaces artificially
arranged to simulate, in the same place, equal special conditions of soil, exposure and slope,
and at the same time, maintaining the equality of soil, different conditions of exposure and
slope. Therefore this study aimed to determine the radiation balance on a horizontal grassy
surface and correlate it with the radiation balance on grassy surfaces with different
expositions and declivities to establish equations that can be used in the reference
evapotranspiration estimates, considering the positioning of the site measured.
2. Material and Methods
The research was developed in a structure called "Experimental Hydrographic Basin", from
the Department of Rural Engineering, FCAV/UNESP, Jaboticabal Campus, SP, described
with details by Turco et al. (1998). In this structure, an experiment was undertaken from
March 2002 to March 2003, in which surfaces of 10.5 m 2 were used, characterized as H
(horizontal), 10N (10% of slope and northern exposure), 20N (20% of slope and northern
exposure), 10S (10% of slope and southern exposure), 20S (20% of slope and southern
exposure), 10E (10% of slope and eastern exposure), 20E (20% of slope and eastern
exposure) , 10W (10% of slope and western exposure) and 20W (20% of slope and western
exposure), that simulates terrain with displays and slope commonly used in agriculture. In the
experimental area surfaces Bahiagrass (Paspalum notatum Flügge) was planted in order to
simulate the conventional weather station areas. The amount of water applied to each
surface was in function of the ETo values obtained by Penman-Monteith method (Allen et al.,
1998), adjusted for each surface. Irrigation was performed in the late afternoon with an
irrigation frequency of one day. The irrigation on each surface was performed through the
installation of six hoses 3.5 m long, perforated every 20 cm, in its entire length. For the
determination of radiation balance on the surface, was installed in the center area of each
surface, a net radiometer (radiation balance sensor, model NRLITE of Kipp & Zonnen),
parallel to the surface totalizing nine equipment. The sensor of each area was fixed in an
aluminum frame, which remained at 1.0 m above the surface. The radiation balance on the
surface was recorded by a data acquisition system, composed by a Datalogger CR10X,
brand Campbell Scientific, Inc., being installed a data acquisition system in each basin. The