In forested catchments, the exceedance of rainfall and antecedent water storage thresholds is often required for runoff generation, yet to our knowledge these threshold relationships remain undescribed in tropical dry forest catchments. We, therefore, identified the controls of streamflow activation and the timing and magnitude of runoff in a tropical dry forest catchment near the Pacific coast of central Mexico. During a 52 day transition phase from the dry to wet season, soil water movement was dominated by vertical flow which continued until a threshold soil moisture content of 26% was reached at 100 cm below the surface. This satisfied a 162 mm storage deficit and activated streamflow, likely through lateral subsurface flow pathways. High antecedent soil water conditions were maintained during the wet phase but had a weak influence on stormflow. We identified a threshold value of 289 mm of summed rainfall and antecedent soil water needed to generate >4 mm of stormflow per event. Above this threshold, stormflow response and magnitude was almost entirely governed by rainfall event characteristics and not antecedent soil moisture conditions. Our results show that over the course of the wet season in tropical dry forests the dominant controls on runoff generation changed from antecedent soil water and storage to the depth of rainfall.

Abstract:Studies of hydrological processes in tropical dry forests are less than 1% of published forest hydrology literature. The strong drywet seasonality typical of the tropical dry forest ecoregion is similar to those that characterize semi-arid regions. In semi-arid systems, infiltration is often limited by low hydraulic conductivities (K) and extreme levels of soil water repellency, which when combined with high rainfall intensities, result in infiltration excess (Hortonian) overland flow (HOF) as a dominant runoff mechanism. Given that little is known about the surface runoff hydrology of tropical dry forests, we tested the hypothesis that our knowledge about controls on runoff in semi-arid systems is transferrable to tropical dry forest hillslopes. Our results show that tropical dry forest soils do develop a strong water repellency during the dry season; however, this does not persist through the wet season. In our period of study, surface K ranged from 9 to 164 mm/h and was greater than the rainfall intensity for more than 75% of the rain events. In our period of study, rainfall intensities were generally low with more than half of the storm events falling between 0.2 and 4.2 mm/h. The low rainfall intensities, high K and lack of repellent surfaces during the wet season result in the percolation of >70% of the annual rainfall through the upper 30 cm of soil, indicating that subsurface flow, not HOF, is the primary runoff mechanism. These findings show that in spite of similar climate and vegetation regimes, hydrological knowledge from semi-arid catchments is not transferrable to tropical dry forests.

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Following harvesting by manual block-cut methods and subsequent abandonment, Cacouna bog has undergone a natural vegetation succession, with ericaceous shrubs covering more than 90% of the surface. The abundance of shrubs plays a major role in the soil water flux and availability at the site, impacting Sphagnum regeneration. From June 1 to August 22, 2007, field measurements indicate that transpiration represented the largest water loss from the shrubs at 1Ð7 mm day 1 , comprising 142 mm (42%) of rainfall, compared to 93 mm of evaporation (28%) from bare soil. The rainfall interception from the canopy (62 mm) and litter (15 mm) accounted for 23% of seasonal rainfall. Thus after transpiration and interception losses are accounted for, only 115 mm of the 334 mm of rain (34%) remained available for other processes (recharge/soil evaporation). In the field, the litter layer prevented 17 mm from being lost over the summer as it reduced evaporation by 18%. Laboratory experiments using intact soil monoliths with and without shrubs and litter indicate that at depths below 10 cm the water content from the shrub monoliths decreased 27% versus 20% in the bare peat monoliths because of root water uptake. As a management prescription, raising the water table within 20 cm of the surface would provide water to the most active root uptake zones, reducing the need for extraction from the upper 10 cm of the peat. At this level sufficient water can be supplied to the surface through capillary rise, providing adequate water for the reestablishment/survival of Sphagnum.