poster - International Conference of Agricultural Engineering

uncovered ones, that mixed the water layers, led covered AWRs to present a slight thermal
gradient during the warmer months (maximum of 2ºC in September). The thermal gradient
disappeared in the fall, when the first autumn rain cooled the upper layer inducing thermal
mixing in the water.
Such small thermal gradient registered on covered reservoirs does not match the results
reported by Maestre-Valero et al. (2011) who observed a thermal gradient of 12ºC between
the deeper and surface layers in water reservoirs no-regulated for irrigation. The use of
reservoirs for irrigation, which implied the inlet and outlet of water and therefore a short time
of permanence of the water in the AWR, was likely to soften the effect of the installation of
the cover on the water thermal stratification.
Electrical conductivity
Unlike what happens in AWRs without regulation (Maestre-Valero et al., 2011), in this study,
where AWRs were used for irrigation, changes in the EC are the result of a water and salt
balance that involves both the water entries as storing and rainfall and the outputs as water
used for irrigation and evaporation. Martínez-Alvarez et al. (2009) indicated that for an
uncovered reservoir of about 12,000 m 3 which supplies an area of about 4 ha, the volume of
water evaporated and rainfall with respect to the regulated water volume was 14.2% and
2.8% respectively. Such values for the same AWR in covered conditions were about 4.0%.
Accordingly, those low percentages indicate that in regulated AWRs, evaporation and rainfall
play a minor role in EC reductions; being EC changes mainly due to water renewals.
Chlorophyll-a (Algae)
In covered AWRs, the cover did not allow the proliferation of algae and hence Chl-a was
rather low during the experimental period (< 1 μg L -1 ). For uncovered AWRs, however, the
incidence of the solar radiation in the water favoured the photosynthesis processes and
hence the algae proliferation. A significant difference in algae concentration was also found
between uncovered AWRs U 1 and U 2 . Water in U 1 remained stagnant for longer periods and
renewals were also less frequent. U 1 reached a maximum of Chl-a of 52 μg L -1 . U 2 renewed
water more frequently only reaching a maximum of 25 μg L -1 . For both uncovered AWRs,
maximums of Chl-a were reached in September when the climatic conditions for algae
growth were more suitable. W t followed the Chl-a trend during the experimental period.
Dissolved oxygen
Maestre-Valero et al. (2011) manifested that the oxygen concentration in the stored water in
an AWR not used for irrigation was almost completely depleted in about two months after
installing a SSCC. Unlike what happens with DO in that kind of AWRs, DO in covered AWRs
with regulation for irrigation, where there is a short time of permanence of the water in the
AWR, remained close to saturation during the whole experimentation period (Fig. 2).
Renewals of water in covered AWRs increased the DO concentration, having this factor a
more important effect in the DO concentration than the reduction of DO by the installation of
the SSCC. DO in uncovered AWRs was slightly higher than DO in covered AWRs. The
continuous renewals of water and the oxygation process by oxygen diffusion on the surface
water and generation by photosynthesis allowed reaching such higher DO concentrations.
3.2. Effects of SSCCs on chemical parameters
The determination of chemical parameters of water for irrigation is of substantial importance.
For instance, Na + , B + and Cl - are toxic to plants and in addition, high concentration of Na +
-
2-
may cause problems of permeability in the soil. NO 3 and SO 4 are on the one hand, a
natural source of nutrients for plants but, instead, present a risk of water eutrophication that
can lead to a risk of clogging drip emitters during the irrigation. Covering AWRs hardly had
consequences in the chemical water quality parameters and statistical analyses indicated
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