To achieve sustainable development and to ensure water availability in hydrological basins, water managers need tools to determine the actual evapotranspiration (ET) on a large scale. Field energy balances from irrigated and natural ecosystems together with a net of agro-meteorological stations were used to develop two models for ET quantification at basin scale, based on the Penman-Monteith equation. The first model (PM1) uses the resistances to the latent heat fluxes estimated from satellite measurements, while the second one (PM2) is based on the ratio of ET to the reference evapotranspiration (ET 0 ) and its relation to remote sensing parameters. The models were applied in the Low-Middle São Francisco river basin in Brazil and, after comparison against field results, showed good agreements with PM1 and PM2 explaining, respectively, 79% and 89% of the variances and mean square errors (RMSE) of 0.44 and 0.34 mm d . Petrolina and Juazeiro, in Pernambuco (PE) and Bahia (BA) states, respectively, were highlighted with the biggest irrigated areas. The highest increments are for vineyards and mango orchards. For the first crop the maximum increment was verified between 2003 and 2004 in Petrolina-PE, when the cultivated area increased 151%. In the case of mango orchards the most significant period was from 2005 to 2006 in Juazeiro-BA (129%). As the best performance was for PM2, it was selected and used to analyse the regional ET at daily and annual scales, making use of Landsat images and a geographic information system for different soil . It could be concluded that irrigated mango orchards and vineyards in that year consumed more water than caatinga by factors of 3 and 2, respectively. The mango orchards and vineyard areas, representing 19.4 and 8.2% of the total irrigated area, respectively, resulting in a total evaporative depletion of 0.22 km 3 yr −1 in the growing regions comprised of the agro-meteorological stations.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright

The intensification of irrigated agriculture in the semi-arid region of Brazilian Northeast results in a change of natural vegetation by irrigated fruit crops. New applications of remote sensing technologies are presented in this paper to estimate the impact of this land use change on regional water consumption-and ultimately the water balance-in Low-Middle Sã o Francisco River basin. Ten Landsat images for a period from 2001 to 2007 were used, together with the locally calibrated Surface Energy Balance Algorithm for Land (SEBAL) and agro-meteorological data to derive information on regional actual evapotranspiration (ET), biomass production (BIO), and crop water productivity (CWP). The Landsat-based results revealed that regional mean ET for irrigated crops was 3.6 mm d À1 being higher than for natural vegetation (1.4 mm d À1). Similar incremental ET values between natural and irrigated ecosystems were found from micro-meteorological field experiments. The consequence of this land use change on Sã o Francisco River's downstream stream flow was assessed by estimating volumetric incremental evapotranspiration at the regional scale. The bio-physical crop water productivity per unit of actual evapotranspiration (CWP ET) varied between 0.4 and 1.7 l of wine per m 3 of water for wine grapes; 1.7 and 4.0 kg of fruits per m 3 of water for table grapes; and 2.2 and 5.0 kg of fruits per m 3 of water for mangos. The accompanying paper (Part A) describes the calibration and validation of SEBAL steps witnessed under the actual field conditions in this study area.

No abstract

Abstract:In the Brazilian semi-arid region, the intensification of agriculture results in a change of natural vegetation by irrigated crops. To quantify the contrast between these two ecosystems, the large-scale values of water productivity components were modelled in Petrolina (PE) and Juazeiro (BA) municipalities. The SAFER (Simple Algorithm For Evapotranspiration Retrieving) algorithm was used to acquire evapotranspiration (ET), while the Monteith's radiation model was applied for estimating the biomass production (BIO). Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images were used together with agro-meteorological data. In Petrolina and Juazeiro, the mean monthly ET values for irrigated crops were 938 and 739 mm·month .Water productivity (WP) was analysed by the ratio of BIO to ET, defined here as the ratio of the net benefits from the mixed agricultural systems to the amount of water required for producing those benefits. The highest incremental WP values, as a result of the irrigated crops introduction, happened outside the rainy period. More spatial WP uniformity occurred in natural vegetation, when comparing with irrigated crops. The most frequent WP values in Petrolina were between 1.6 and 2.2 kg·m . The differences between the municipalities can be mainly explained by differences in precipitation and soil OPEN ACCESSRemote Sens. 2013, 5 5784 water storages conditions, promoting better rainfall use efficiency by the natural vegetation in the first one. The results of the current research are important for appraising the land use change impacts in situations of expanding irrigation areas.

Crop water parameters, including actual evapotranspiration, transpiration, soil evaporation, crop coefficients, evaporative fractions, aerodynamic resistances, surface resistances and percolation fluxes were estimated in a commercial mango orchard during two growing seasons in Northeast Brazil. The actual evapotranspiration (E a) was obtained by the eddy covariance (EC) technique, while for the reference evapotranspiration (E 0); the FAO Penman-Monteith equation was applied. The energy balance closure showed a gap of 12%. For water productivity analysis the E a was then computed with the Bowen ratio determined from the eddy covariance fluxes. The mean accumulated E a for the two seasons was 1419 mm year À1 , which corresponded to a daily average rate of 3.7 mm day À1. The mean values of the crop coefficients based on evapotranspiration (K c) and based on transpiration (K cb) were 0.91 and 0.73, respectively. The single layer K c was fitted with a degree days function. Twenty percent of evapotranspiration originated from direct soil evaporation. The evaporative fraction was 0.83 on average. The average relative water supply was 1.1, revealing that, in general, irrigation water supply was in good harmony with the crop water requirements. The resulting evapotranspiration deficit was 73-95 mm per season only. The mean aerodynamic resistance (r a) was 37 s m À1 and the bulk surface resistance (r s) was 135 s m À1. The mean unit yield was 45 tonne ha À1 being equivalent to a crop water productivity of 3.2 kg m À3 when based on E a with an economic counterpart of US$ 3.27 m À3. The drawback of this highly productive use of water resources is an unavoidable percolation flux of approximately 300 mm per growing season that is detrimental to the downstream environment and water users.

There is a growing interest in quantifying regional scale actual evapotranspiration (ET) for water accounting and for water productivity assessments at river basin scale. Methods that provide point values fail to describe the situations at larger scales. Remote sensing measurements can be used at different spatial scales. This paper applies the theory of the Surface Energy Balance Algorithm for Land (SEBAL). SEBAL was originally derived for Egypt, Spain and Niger [Bastiaanssen, W.G.M., 1995. Regionalization of surface flux densities and moisture indicators in composite terrain: a remote sensing approach under clear skies in Mediterranean climates. Ph.D. dissertation, CIP Data Koninklijke Bibliotheek, Den Haag, The Netherlands. 273 pp.] and was calibrated and validated using ground measurements from four flux sites and from seven agro-meteorological stations in the semi-arid region of the Low-Middle Sã o Francisco River basin, Brazil. Measured parameters included surface albedo, surface temperature, atmospheric and surface emissivity, soil heat flux, surface roughness, net radiation, air temperature gradients, sensible heat flux, latent heat flux, evaporative fraction, and photosynthetically active radiation. The daily ET was estimated (RMSE of 0.38 mm d À1) for mixed agricultural and natural ecosystems. The improved coefficients for the local conditions can now be used to study the impact of expanding irrigated agriculture on the regional water balance and to quantify the water productivity of irrigated horticulture that is the largest water consumer in the Brazilian semi-arid region. Both applications are described in an accompanying paper (Part B).

The energy balance (EB) components were quantified in a commercial farm with corn crop, irrigated by central pivots, in the Northwestern side of São Paulo state, Southeast Brazil. The SAFER (Simple Algorithm For Evapotranspiration Retrieving) was applied to retrieve the latent heat flux (λE), considering six pivots, covering irrigated areas from 74 to 108 ha. With λE quantified and considering soil heat flux (G) as a fraction of net radiation (R n), the sensible heat flux (H) was acquired as a residual in the energy balance equation. Seven Landsat satellite images, covering all corn crop stages from 23 April 2010 to 29 August 2010, allowed relating the energy balance components according to the accumulated degree-days (DD ac) from the planting to harvest dates. The average R n values ranging from 5.2 to 7.2 MJ m-2 day-1 , represented 30 to 45% of global solar radiation (R G). Considering the variation of the energy balance components along the corn crop growing seasons, the average ranges for λE, H and G were respectively 0.0 to 6.4 MJ m-2 day-1 ,-1.5 to 6.7 MJ m-2 day-1 and 0.1 to 0.6 MJ m-2 day-1. The fraction of the available energy (R n-G) used as λE was from 0.0 to 1.3 indicated a good irrigation management, insuring that the water deficit could not be the reason of any yield reduction. Although R n did not reflected well the crop stages, its partition strongly depended on these stages. λE higher than R n and the negative H/R n , happened sometimes along the corn growing seasons. This occurred after the vegetative growth and before the harvest times, indicating heat advection from the surrounding areas to the irrigation pivots, which represented an additional energy source for the evaporative process. The models applied here with only the visible and infrared bands of the Landsat sensor are very useful for the energy balance analyses, considering the size of the corn crop irrigation pivots in Southeast Brazil, when subsidizing a rational irrigation water application in corn crop.