DIFFERENT METHODS FOR MEASURING EVAPORATION IN A TROPICAL RESERVOIR: THE CASE OF THE GAVIÃO RESERVOIR IN THE STATE OF CEARÁ

FEITOSA, GABRIELA PINHEIRO; ARAÚJO, JOSÉ CARLOS DE; BARROS, MÁRIO UBIRAJARA GONÇALVES

doi:10.1590/1983-21252021v34n217rc

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Agricultural Engineering • Rev. Caatinga 34 (2) • Apr-Jun 2021 • https://doi.org/10.1590/1983-21252021v34n217rc copy

DIFFERENT METHODS FOR MEASURING EVAPORATION IN A TROPICAL RESERVOIR: THE CASE OF THE GAVIÃO RESERVOIR IN THE STATE OF CEARÁ

EVAPORAÇÃO EM RESERVATÓRIO TROPICAL MEDIDA POR DIFERENTES MÉTODOS: O CASO DO AÇUDE GAVIÃO, CE

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT

Studies that allow the estimation of evaporation in reservoirs represent an important action for the adequate management of water resources. Thus, this study aimed at estimating evaporation in the tropical reservoir Gavião, located in the municipality of Pacatuba, Ceará, Brazil, and to verify the effect of these estimates on the water availability of the reservoir using the VYELAS model. The results of Penman's methods and the Water Balance were compared with the values obtained from the hydrostatic pressure sensor, the most accurate. It was possible to verify that, in relation to the pressure difference method, all the conventional methods overestimate the evaporation in the reservoir. The method that presented estimates closer to those obtained by the pressure difference sensor was that of Penman, based on data from an onboard station in the lake, with a deviation of only 12%. The method of water balance also presented reliable results for monthly average. The two methods usually accepted in the evaporation calculation (water balance ignoring the infiltration; and Penman's method for meteorological station data on land distant from the lake) presented the most disturbing evaporation values significantly altering the water availability. The results of the VYELAS model showed that evaporated flows, estimated by several methods, exceeded the reference flow by up to 83%. The results demonstrate the great sensitivity of the estimate of water availability in relation to the evaporation rate in the lake.

Keywords:
Hydrology of lentic environments; Water balance of semi-arid regions; VYELAS Model

Station	Latitude (degrees)	Longitude (degrees)	Altitude (m)	Distance from the surface of the water
Onboard automatic station	-3.908	-38.573	37	On the water (zero)
Nearby automatic station	-3.902	-38.564	52	400 m
Distant automatic station	-3.745	-38.581	20	19,000 m

Characteristic	Value
Hydrographic basin (km²)	97
Residence time (year)	1.02
Storage capacity (hm³)	33.3
Form coefficient, α (Equation 10)	17927
Mean annual inflow rate (hm³.yr^-1)	32.6
Coefficient of variation of annual outflow	0.80
Length of the dry season (number of days)	214

Method	Rate of evaporation (mm)
Method	Daily Average	Total for the period
Differential pressure sensor method	6.9	395
Penman method – Onboard station	7.7^ns	478
Penman method – Shore station	8.2^* *The difference between the mean value for the method and for the reference method is significant by Student's t-test at 5% probability, ns the mean value for the method and for the reference method do not differ significantly by Student's t-test at 5% probability.	508
Penman method – Station on the Campus do Pici	11.8^* *The difference between the mean value for the method and for the reference method is significant by Student's t-test at 5% probability, ns the mean value for the method and for the reference method do not differ significantly by Student's t-test at 5% probability.	734
Water balance method (maximum rate)	13.7^* *The difference between the mean value for the method and for the reference method is significant by Student's t-test at 5% probability, ns the mean value for the method and for the reference method do not differ significantly by Student's t-test at 5% probability.	853
Water balance method (minimum rate)	9.2	569

Method	Index				Error
Method	R	R²	d	c	ME	RMSE	SEE	CRM
E_PN–ONB	0.155	0.024	0.431	0.067	-0.779	2.406	0.793	-0.113
E_PN–BNK	0.149	0.022	0.441	0.066	-1.264	2.287	1.287	-0.183
E_PN-PC	0.110	0.012	0.340	0.037	-4.955	5.378	5.043	-0.716
WB_MAX	-0.032	0.001	0.058	-0.002	-9.701	34.723	9.875	-1.401
WB_MIN	-0.032	0.001	0.083	-0.003	-4.160	22.674	4.234	-0.601

Method	Dry-season evaopration (m)
Differential pressure sensor method	1.48
Penman method – AWS Onboard station	1.65
Penman method – AWS Shore of the reservoir	1.75
Penman method –CWS Campus do Pici	2.53
Water balance method (maximum rate)	2.95
Water balance method (mimimum rate)	1.97

Method for calculating evaporation ^[] []** QW0 = water availability calculated based on the reference data; QED = mean evaporated flow rate during the dry season; QED0 = evaporated flow rate during the dry season calculated based on the reference data; QO = spill rate (bleed rate).	Q_W	$\overset{─}{Q_{W}}$	$\overset{─}{Q_{ED}}$	$\overset{─}{Q_{0}}$	Q_W/Q_W0	Q_ED/Q_ED0
Differential pressure sensor	5.290	5.264	9.329	17.621	100%	100%
Penman - AWS onboard station	4.580	4.558	10.316	17.484	87%	111%
Penman - AWS Shore of the reservoir	4.240	4.219	10.868	17.386	80%	116%
Penman method -CWS Campus do Pici	0.840	0.836	15.150	16.172	16%	162%
Water balance (maximum rate)	0.019	0.019	17.107	15.800	0.4%	183%
Water balance (mimimum rate)	3.210	3.195	12.145	17.406	61%	130%

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