To ensure a safe drinking water supply, it is necessary to protect water quality. To classify the suitability of the Orós Reservoir (Northeast of Brazil) water for human consumption, a Water Quality Index (WQI) was enhanced and refined through a Principal Component Analysis (PCA). Samples were collected bi-monthly at seven points (P1 – P7) from July 2009 to July 2011. Samples were analysed for 29 physico-chemical attributes and 4 macroinvertebrate metrics associated with the macrophytes Pistia stratiotes and Eichhornia crassipes. PCA allowed us to reduce the number of attributes from 33 to 12, and 85.32 % of the variance was explained in five dimensions (C1 – C5). Components C1 and C3 were related to water-soluble salts and reflect the weathering process, while C2 was related to surface runoff. C4 was associated with macroinvertebrate diversity, represented by ten pollution-resistant families. C5 was related to the nutrient phosphorus, an indicator of the degree of eutrophication. The mean values for the WQIs ranged from 49 to 65 (rated as fair), indicating that water can be used for human consumption after treatment. The lowest values for the WQI were recorded at the entry points to the reservoir (P3, P1, P5, and P4), while the best WQIs were recorded at the exit points (P6 and P7), highlighting the reservoir’s purification ability. The proposed WQI adequately expressed water quality, and can be used for monitoring surface water quality.

Quantifying the organic carbon content of soil over large areas is essential for characterising the soil and the effects of its management. However, analytical methods can be laborious and costly. Reflectance spectroscopy is a well-established and widespread method for estimating the chemical-element content of soils. The aim of this study was to estimate the soil organic carbon (SOC) content using hyperspectral remote sensing. The data were from soils from two localities in the semi-arid region of Brazil. The spectral reflectance factors of the collected soil samples were recorded at wavelengths ranging from 350–2500 nm. Pre-processing techniques were employed, including normalisation, Savitzky–Golay smoothing and first-order derivative analysis. The data (n = 65) were examined both jointly and by soil class, and subdivided into calibration and validation to independently assess the performance of the linear methods. Two multivariate models were calibrated using the SOC content estimated in the laboratory by principal component regression (PCR) and partial least squares regression (PLSR). The study showed significant success in predicting the SOC with transformed and untransformed data, yielding acceptable-to-excellent predictions (with the performance-to-deviation ratio ranging from 1.40–3.38). In general, the spectral reflectance factors of the soils decreased with the increasing levels of SOC. PLSR was considered more robust than PCR, whose wavelengths from 354 to 380 nm, 1685, 1718, 1757, 1840, 1876, 1880, 2018, 2037, 2042, and 2057 nm showed outstanding absorption characteristics between the predicted models. The results found here are of significant practical value for estimating SOC in Neosols and Cambisols in the semi-arid region of Brazil using VIS-NIR-SWIR spectroscopy.

The objective of this research was to define the potential of brackish groundwater for 15 systems of biosaline agriculture in a representative area of the Brazilian semi-arid region. The study was conducted using a database of the State of Ceará, with 6284 wells having brackish water (EC ≥ 0.8 dS m⁻¹ and discharge rate ≥ 0.5 m³ h⁻¹). Our results show that the potential of brackish groundwater resources depends on the set of data: (i) production system (crop salt tolerance and water demand) and (ii) water source (salinity and well discharge rate). The joint analysis of these data shows that plant production systems with lesser water requirements, even with moderate tolerance levels to salt stress, present better results than more tolerant species, including halophytes and coconut orchards. About 41, 43, 58, 69, and 82% of wells have enough discharge rates to irrigate forage cactus (1.0 ha), sorghum (1.0 ha with supplemental irrigation), hydroponic cultivation, cashew seedlings, and coconut seedlings, respectively, without restrictions in terms of salinity. Otherwise, 65.8 and 71.2% of wells do not have enough water yield to irrigate an area of 1.0 ha with halophytes and coconut palm trees, respectively, butmore than 98.3 and 90.7% do not reach the water salinity threshold for these crops. Our study also indicates the need for diversification and use of multiple systems on farms (intercropping, association of fish/shrimp with plants), to reach the sustainability of biosaline agriculture in tropical drylands, especially for family farming.

The rural population of a tropical semi-arid region such as the northeast of Brazil is directly dependent on rainfall for agricultural production, pasture for livestock, and the storage of drinking water for human consumption. The region is characterised by the frequent occurrence of dry years and consecutive dry days (CDD) during the rainy season, demonstrating the vulnerability of rainfed agriculture. The aim of this study therefore, was to identify homogeneous regions of the State of Ceará where there is greater or lesser uncertainty of the rainfall regime. Accordingly, the occurrence of dry and very dry years were investigated as uncertainties between years, together with the occurrence of two classes of CDD (10-20 and 21-30 days) during the rainy season. To define similar regions as to the uncertainty of the rainfall regime, the technique of Hierarchical Cluster Analysis (HCA) was used. The study considered time series of least 30 years, from 77 rain gauge stations around the state of Ceará. The use of HCA resulted in the formation of seven homogenous groups in both of the phenomena being investigated. Municipalities in the Central Backlands, the Inhamuns Backlands and the Jaguaribana Region displayed a higher frequency of both inter-annual (dry years) and intra-annual (CDD) uncertainty, demonstrating the greater vulnerability of rainfed agriculture. The occurrence of a CDD event is not dependent on the total annual rainfall, as several stations with an average rainfall higher than that of the state showed potential for the occurrence of CDD. The number of CDD events recorded in the 11-20 day class was higher than in the 21-30 day class.

Cotton yield under the rainfed farming system in tropical semi-arid regions is significantly decreased by prolonged dry spells. In this context, supplemental irrigation (SI) with treated wastewater emerges as a strategy for the sustainability of agricultural production besides contributing to the reduction of fertilizer costs. The objective of this research was to evaluate the productivity, photosynthetic parameters, fiber quality, and profitability of cotton cultivation, under supplemental irrigation (SI) with municipal treated wastewater (MTW), with and without mineral fertilization. The treatments consisted of three water scenarios (normal, drought, and severe drought), defined by the historical series of precipitation data of 30 years, and two treatments of supplemental irrigation with MTW, with or without NPK fertilization. SI with treated wastewater increased cotton yield in all three scenarios (normal, drought, and severe drought) by approximately 29%, 255%, and 251%. Longer dry spells in drought and severe drought scenarios increased the volume of SI with MTW, resulting in greater nutrient input to the soil, improved photosynthetic response, higher physical water productivity, reduction in fertilizer costs, and higher farmers’ income. Therefore, our results show the importance of using treated wastewater in supplemental irrigation of cotton under scenarios of water scarcity in tropical semi-arid regions.