INCREASE IN WATER-SCARCITY RISK IN A BRAZILIAN DRY-REGION RESERVOIR

GIL, MARGARITA MARIA LÓPEZ; ARAÚJO, JOSÉ CARLOS DE; MONTENEGRO, SUZANA MARIA GICO LIMA; VALENÇA, JULIANA MARIA DE MORAIS SOBREIRA

doi:10.1590/1983-21252020v33n418rc

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Agricultural Engineering • Rev. Caatinga 33 (4) • Oct-Dec 2020 • https://doi.org/10.1590/1983-21252020v33n418rc copy

INCREASE IN WATER-SCARCITY RISK IN A BRAZILIAN DRY-REGION RESERVOIR

AUMENTO DO RISCO DE ESCASSEZ HÍDRICA EM UM RESERVATÓRIO DA REGIÃO SECA BRASILEIRA

Authorship SCIMAGO INSTITUTIONS RANKINGS

ABSTRACT

This study assesses the extent to which silting increases water-scarcity risk, considering the temporal reduction of water availability and increased demand using land use and water-demand scenarios at the transition of Caatinga and Cerrado biomes of the Bocaina reservoir watershed (10³hm⁴), in the Brazilian dry region. Methodological steps were: reservoir silting measured in-situ 20 years after dam construction; climate variables computed with the aid of a conventional station (2005-2014); soil erodibility assessed using 16 soil samples; and topography and land cover estimated based on 21 years of Landsat imagery. Three land use scenarios were generated (invariability, degradation and preservation) with the climate scenario derived from the semi-arid rainfall temporal variability; whereas two water-demand scenarios (invariability and higher efficiency) were a function of the efficiency of the irrigation systems. Water availability was calculated using the volume-yield elasticity (VYELAS) Model. The field results (1985-2015) showed a gross erosion rate of 13.5 Mg-ha^-1 -yr^-1 in the basin. The annual sediment yield (1.7 Mg-ha^-1) and the decadal reservoir silting (1.0%) were below regional average due to the low sediment delivery ratio (12.6%) in the area. Scenario projections (2040) suggest water demand may double if irrigation methods do not improve, whereas siltation may cause water availability to decrease up to 10% in the period. In this case, the water-supply reliability will be below the recommended standard value (90%), regardless of the land use scenario. Nevertheless, simultaneous soil preservation and improved irrigation efficiency can reduce the decadal water-scarcity risk from 82% (worst scenario) to 17%.

Keywords:
Caatinga biome; Cerrado biome; Water management; Water demand; Silting

Data	Source	Values
River inflow: annual average and variation coefficient (CV).	Hydrographs (period 2000-2015) from registered data by DNOCS-PI (Departamento Nacional de Obras Contra as Secas).	Annual average = 69.4 hm³ CV = 0.40
Morphological coefficient α and reservoir storage capacity in the construction year (1985).	PIAUÍ (2016b). Height-area-volume curves.	Coefficient α = 1,667 Capacity = 115 hm³
Morphological coefficient α and reservoir storage capacity in the bathymetric survey year (2015).	PIAUÍ (2016a). Height-area-volume curves.	Coefficient α = 2,197 Capacity = 111 hm³
Evaporation rate of the dry season (CAMPOS, 2010CAMPOS, J. N. B. Modeling the yield-evaporation-spill in the reservoir storage process: The regulation triangle diagram. Water Resources Management, 24: 3487-3511, 2010.)	Class-A pan from Picos Climate Station operated by Instituto Nacional de Meteorologia (INMET). For dryland reservoirs, multiply class-A pan by 0.70 - in Mamede et al.(2012)MAMEDE, George L. et al. Overspill avalanching in a dense reservoir network. Proceedings of the National Academy of Sciences, 109: 7191-7195, 2012. Data period:1976 - 1990	In the dry season, the average evaporation rate is 1.72 m per year

Soil class	Average K (Mg·h·MJ^-1·mm^-1)	Area (%)
Argisol	0.052	10.5
Yellow Latosol	0.028	44.8
Litholic Neosol	0.042	39.7
Quartzarenic Neosol	0.034	5.0
Weighted average	0.036

	1985	2015	Φ (-)	2040
	1985	2015	Φ (-)	S1	S2	S3
Erosion ε (Mg∙ha^-1)	(-)	13.5	(-)	13.5	28.1	8.6
Sediment yield (Mg∙ha^-1)	(-)	1.7	(-)	1.7	3.5	1.1
Silting since 1985 (hm³)	(-)	3.6	(-)	7.3	11.9	6.1
Silting since 1985 (%)	(-)	3.2	(-)	6.3	10.4	5.3
Storage capacity (hm³)	115.0	111.4	(-)	107.7	103.1	108.9
Qw [G = 99%] (hm³)	41.88	41.20	0.519	40.50	39.63	40.73
Qw [G = 95%] (hm³)	48.20	46.70	0.994	45.16	43.24	45.66
Qw [G = 90%] (hm³)	52.11	50.34	1.085	48.52	46.26	49.11
Qw [G = 80%] (hm³)	57.51	55.94	0.872	54.33	52.32	54.85
Qw [G = 70%] (hm³)	62.28	60.51	0,908	58.69	56.43	59.28

Demand	2010	2040
Demand	2010	D1	D2
Human consumption	0.0	9.5	9.5
Animal consumption	0.2	0.4	0.4
Irrigation	20.9	36.2	28.5
Ecological discharge	3.7	3.7	3.7
Total	24.8	49.8	42.1

Scenarios	S1 (invariability)	S2 (degradation)	S3 (preservation)
	Annual demand reliability in 2040
D1 (invariability)	0.88	0.84	0.89
D2 (enhanced irrigation efficiency)	0.98	0.96	0.98
	Risk of decadal water scarcity in 2040
D1 (invariability)	0.72	0.82	0.68
D2 (enhanced irrigation efficiency)	0.20	0.32	0.17

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E-mail: caatinga@ufersa.edu.br

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