Water demand and flows in the Sa ˜o Francisco River Basin (Brazil) with increased irrigation M.P. Maneta a, *, M. Torres b , W.W. Wallender a , S. Vosti b , M. Kirby c , L.H. Bassoi d , L.N. Rodrigues e a Department of Land, Air and Water Resources, University of California, Davis, One Shields Avenue, 95616, Davis, CA, United States b Department of Agricultural Resource Economics, University of California, Davis, One Shields Avenue, 95616, Davis, CA, United States c CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia d Embrapa Semia ´rido, BR 428, km 152, 56302-970 Petrolina, PE, Brazil e Embrapa Cerrados, BR 020, km 18, 73310-970 Planaltina, DF, Brazil 1. Introduction The Sa ˜o Francisco River has the largest basin fully contained within Brazil. It is located in the northeast part of the country covering about 630,000 km 2 . The river flows south to north along 2860 km, covering diverse climatic regions. The population living within the territory drained by the river is about 13 million, with a large percentage living in the southern part and especially around the Belo Horizonte metropolitan area. Due to its strategic importance, the Sa ˜o Francisco River Basin has been a focus in political debate. Not only because of its importance as a main route of transportation and communication between the coast and the interior, its hydropower potentials and its potential for irrigation but also because of the dangers posed by floods, droughts or those derived from environmental degradation, proper management of the basin is necessary. However, most of the actions taken in the past have focused on single specific sectors. The partial management of the basin may have had significant negative impacts on other water users because integrated mana- gement was inadequate in terms of harmonizing water demand with the potential supplies (Romano and Cadavid Garcia, 1999). Only from 1997 a federal law provides a legal framework for integrated management under the control of the National Water Agency of Brazil (ANA) and a Commission for the Sa ˜o Francisco River Basin that includes the participation of the public sector at federal, state and municipal levels as well as a wide range of water users and civil organizations (Braga and Lotufo, 2008). Within the basin, irrigation has been seen as one of the greatest opportunities to foster economic and social development, espe- cially in the semiarid northeast. According to the local develop- ment agency in the Sa ˜o Francisco River (CODEVASF), from the 1960s to 1975 the irrigated area has increased steadily at about Agricultural Water Management 96 (2009) 1191–1200 ARTICLE INFO Article history: Received 23 September 2008 Accepted 8 March 2009 Available online 15 April 2009 Keywords: Water management Large basin modeling Water budget Monte Carlo simulations ABSTRACT Most activities that support economic growth in the Sa ˜o Francisco River Basin (Brazil) need water. Allocation of the water resources to each competing use needs quantification in order to develop an integrated water management plan. Irrigation agriculture is the largest water consuming activity in the basin. It has produced large economic and social advancements in the region and has potential for further development. The local development agency in the Sa ˜o Francisco River has projected an increase of more than 500,000 ha in irrigation developments distributed within the basin. Water requirements of the projected irrigation expansions and their effects on river flow were quantified. A semi-distributed model was constructed to simulate the water balance in 16 watersheds within the basin. The watersheds were hydrologically characterized by the average precipitation, atmospheric demand and runoff as well as their variability. Water requirements for increased irrigated agriculture were calculated using an agronomic mass balance. A Monte Carlo procedure generated the variability of irrigation requirements and resulting decreased river flows from the multidimensional probability distribution of the hydrologic variables of each watershed. Irrigation requirements were found to be more variable during the wet season because of weather variability. In contrast to what might be expected, in drier years, irrigation requirements were often larger during the wet season than in the dry season because the cropped area is largest in the wet months and variability of precipitation is greater. Increased irrigation shifted downward the distribution of river flows but not enough to affect other strategic water uses such as hydropower. Further irrigation expansion may be limited by wet season flows. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 530 752 7415; fax: +1 530 752 5262. E-mail address: mpmaneta@ucdavis.edu (M.P. Maneta). Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat 0378-3774/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2009.03.008