Left high and dry: a call to action for increased hydrological research in tropical dry forests Kegan K. Farrick 1 * and Brian A. Branfireun 2 1 Department of Earth Sciences, Western University, London, Ontario, Canada 2 Department of Biology and Centre for Environment and Sustainability, Western University, London, Ontario, Canada *Correspondence to: Kegan K. Farrick, Department of Earth Sciences, Western University, London, Ontario, Canada E-mail: kfarrick@uwo.ca Introduction Tropical dry forests have lived in the shadow of their humid counterparts with respect to scientific research. Despite accounting for more than 42% of all tropical forest (Murphy and Lugo, 1995) and roughly 6% of the Earth’s land surface, less than 15% of the literature on all tropical forest research has focused on tropical dry forests with the remainder highlighting work in tropical wet forests (Sánchez-Azofeifa et al., 2005; Santos et al., 2011). Many of these tropical dry forest regions are currently water stressed, and additional pressures from population growth, land use and future climate change will have significant implications for the future functioning of their natural and socioeconomic systems. In spite of the recognized importance of water resources in the tropics, there have been remarkably few publications on the hydrological processes in this region as compared to the humid temperate zone (e.g. Bonell, 1993, 1998; Buttle et al., 2009; Ohte and Tokuchi, 2011; Price, 2011). This is especially true for tropical dry forests, which is surprising given that surface runoff is often limited to a 3 – 5 month period annually, with an intervening dry period with little to no precipitation (Ortiz-Jiménez et al., 2005; Love et al., 2010). Hence, there is a critical need to understand the hydrology of tropical dry forests, particularly in light of current land use and future climate change issues facing these regions. In this, commentary we will highlight the: 1. global distribution, diversity and threats to tropical dry forests; 2. current state of knowledge of the hydrological processes in tropical dry forest catchments, and; 3. hydrological research gaps and priorities for tropical dry forests. Topical Dry Forest Distribution and Assessment of Major Threats Tropical dry forests lie within the tropical zone, which extends from the equator to 23° in both the Northern and Southern hemispheres (Figure 1). They are broadly characterized as having a vegetation community typically dominated by deciduous to semi-deciduous trees, annual precipitation ranges from 250 to 2000 mm, average annual temperature is greater than or equal to 17 °C and an annual average ratio of potential evapotranspiration (PET) to precipitation (P) greater than 1 (Murphy and Lugo, 1995). The key defining feature of tropical dry forests is the occurrence of a distinct dry period that lasts between 3 and 7 months (Bullock et al., 1995). This ecosystem accounts for more than 42% of tropical forests and 19% of the world’s total forest area (Murphy and Lugo, 1995; Miles et al., 2006). The majority of tropical dry forests occur in Central and South America (66.7%) with the remainder found in Asia (16.4%), Africa (13.1%) and small fragments in Oceania (3.8%) (Figure 1). These forests are amongst the most diverse and complex in the world, displaying a high degree of endemism (Olson and Dinerstein, 1998; Suazo- Received 6 June 2013 Accepted 7 June 2013 HYDROLOGICAL PROCESSES Hydrol. Process. 27, 3254–3262 (2013) Published online 23 July 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/hyp.9935 3254 Copyright © 2013 John Wiley & Sons, Ltd.