Runoff Generation In A Tropical Dry Forest Watershed: Processes, Patterns And Connectivity

The lack of understanding regarding the controls that govern runoff generation in tropical dry forests represent a critical gap in the hillslope and catchment hydrology literature. Tropical dry forests account for approximately 42% of the global tropical forests, but represent less than 1% of the forest hydrology literature. Three complementary studies were undertaken in a small tropical dry forest watershed, Mexico, to assess the controls that govern the retention and release of a rainfall in the catchment as runoff. In the first study, the high soil surface hydraulic conductivities, absence of a water repellent surface and low rainfall intensities during the wet season allows most of the incoming rainfall to percolate through the near-surface soil layers, suggesting that runoff is generated through a subsurface flow mechanism. In the second study, it was found that two different thresholds were required for streamflow activation and stormflow generation. The long dry period depletes the stores of soil water. Only after the soil storage deficit in the upper metre is satisfied, is streamflow activated from the catchment. Once streamflow became persistent, the stormflow response was almost entirely governed by the rainfall event characteristics and not antecedent soil moisture conditions. The third study used a combination of isotopic, geochemical and hydrometric measurements to describe the water flow pathways, source areas and residence times of stream water in this catchment. It was shown that runoff produced during storm events were composed primarily of old water that likely originated from either deep subsurface soil layers or groundwater and the source areas expanded, likely through sub-basin connectivity, as catchment wetness increased through the wet season. Given the arid climate of the watershed and known hydrological literature regarding runoff generation in tropical forests, it was hypothesised that runoff in this catchment should be delivered from surface or near-surface sources. However, this dissertation has shown that the combination of deep, permeable soil on steep slopes have a stronger influence on runoff generation in this catchment than climate. These three studies have therefore demonstrated the importance of characterising the physical controls that govern runoff generation in forests that are data poor

The Role of Ericaceous Shrubs in the Surface Water Balance and Soil Water Availability of Cutover Peatland, Québec

Peatlands are carbon sinks and occupy approximately 13% of Canada’s terrestrial surface of which 0.02% have been harvested for horticultural peat. The extraction of peat from natural peatlands alters the hydrology which affects the growth and survival of Sphagnum the primary peat forming vegetation. Ericaceous shrubs do not require specific water content and soil water pressure conditions for their existence and in cutover peatlands they occupy more than 70% of the surface. Rainfall interception, transpiration and root water uptake and redistribution are processes that alter water availability at the soil surface. The high abundance of shrubs at cutover peatlands will influence the surface water balance and more importantly soil water availability and, inevitably determine the success of Sphagnum reestablishment at the site. This study seeks to understand the role that ericaceous shrubs play in the surface hydrological balance at a cutover site and how these changes impact Sphagnum development. Rainfall interception, transpiration, litter layer evaporation and soil water flux was investigated in the field and lab. Approximately 334 mm of rainfall was measured over the season. The shrub canopy and litter had a maximum storage capacity of 4 and 1.2 mm respectively and intercepted ca. 36.7% (120 mm) of rainfall over the season. The effects of rainfall intensity and duration were more important than gross rainfall in determining the amount of water intercepted by the canopy, while the thickness and mass of the litter layer largely determined the storage capacity. Evapotranspiration from shrubs averaged 2.5 mm day-1 with a total of 211 mm over the season. Transpiration was 68% (142 mm) of total evapotranspiration losses, and represented the greatest water loss from the shrub canopy. From these analyses only 22% (72 mm) of rainfall is available for other soil processes and moss development. The evaporation under a litter cover is lower than bare peat and in the field represents water storage of 17 mm over the season. Reduced water input by litter interception is offset by the increased water storage under the litter. Laboratory analyses of soil water flux under ericaceous shrubs show that water loss under the shrubs was greater than bare peat. Water use under the shrubs was highest between -10 and -30 cm and was ca. 2 times greater than bare peat at the same levels. Volumetric water content (θ) decreased throughout the day and water use by shrubs during the day was twice that at night. The shrubs also maintained θ and soil water pressure (ψ) above the threshold values of 50% and -100 cm, respectively, needed for Sphagnum survival. Based on these analyses the shrubs will be beneficial to Sphagnum reestablishment and survival once the primary water losses have been compensated. I recommend raising the water table above -20 cm. In peatland restoration activities, at this level, water used between -10 and -30 cm can quickly be recharged and surface moisture maintained above threshold by capillary rise helping to offset water loss by interception and transpiration through capillary rise

Effects of conversion of native cerrado vegetation to pasture on soil hydro-physical properties, evapotranspiration and streamflow on the Amazonian agricultural frontier

Understanding the impacts of land-use change on landscape-hydrological dynamics is one of the main challenges in the Northern Brazilian Cerrado biome, where the Amazon agricultural frontier is located. Motivated by the gap in literature assessing these impacts, we characterized the soil hydro-physical properties and quantified surface water fluxes from catchments under contrasting land-use in this region. We used data from field measurements in two headwater micro-catchments with similar physical characteristics and different land use, i.e. cerrado sensu stricto vegetation and pasture for extensive cattle ranching. We determined hydraulic and physical properties of the soils, applied ground-based remote sensing techniques to estimate evapotranspiration, and monitored streamflow from October 2012 to September 2014. Our results show significant differences in soil hydro-physical properties between the catchments, with greater bulk density and smaller total porosity in the pasture catchment. We found that evapotranspiration is smaller in the pasture (639 ± 31% mm yr-1) than in the cerrado catchment (1,004 ± 24% mm yr-1), and that streamflow from the pasture catchment is greater with runoff coefficients of 0.40 for the pasture and 0.27 for the cerrado catchment. Overall, our results confirm that conversion of cerrado vegetation to pasture causes soil hydro-physical properties deterioration, reduction in evapotranspiration reduction, and increased streamflow

A call to action for climate change research on Caribbean dry forests

The final publication is available at Springer via https://doi.org/10.1007/s10113-018-1334-6Tropical dry forest (TDF) is globally one of the most threatened forest types. In the insular Caribbean, limited land area and high population pressure have resulted in the loss of over 60% of TDF, yet local people’s reliance on these systems for ecosystem services is high. Given the sensitivity of TDF to shifts in precipitation regimes and the vulnerability of the Caribbean to climate change, this study examined what is currently known about the impacts of climate change on TDF in the region. A systematic review (n = 89) revealed that only two studies addressed the ecological response of TDF to climate change. Compared to the rapidly increasing knowledge of the effects of climate change on other Caribbean systems and on TDF in the wider neotropics, this paucity is alarming given the value of these forests. We stress the need for long-term monitoring of climate change responses of these critical ecosystems, including phenological and hotspot analyses as priorities

Daily soil moisture response to rainfall - Natural Forest and Exotic Pines

This data shows the daily soil moisture response to rainfall at a Natural Secondary Forest and Exotic Pine Planation in Trinidad and Tobago. The soil moisture is measured at 10, 20, 40 and 80 cm below the surface in each forest. The soil moisture at each depth is the determined from the average of three locations in each forest. The soil moisture is measured as the volumetric water content (%). The data also has the daily rainfall in mm

Daily soil moisture response to rainfall - Natural Forest and Exotic Pines

This data shows the daily soil moisture response to rainfall at a Natural Secondary Forest and Exotic Pine Planation in Trinidad and Tobago. The soil moisture is measured at 10, 20, 40 and 80 cm below the surface in each forest. The soil moisture at each depth is the determined from the average of three locations in each forest. The soil moisture is measured as the volumetric water content (%). The data also has the daily rainfall in mm

Hydraulic conductivity data in Native forest and Pine plantation

Unsaturated hydraulic conductivity data for Native forest and pine planations during the start of dry, mid dry, start of wet and mid wet seasons. The data was collected at 40 locations in each site. The data was sampled at the same location for each season.THIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV

Daily soil moisture response to rainfall - Natural Forest and Exotic Pines

This data shows the daily soil moisture response to rainfall at a Natural Secondary Forest and Exotic Pine Planation in Trinidad and Tobago. The soil moisture is measured at 10, 20, 40 and 80 cm below the surface in each forest. The soil moisture at each depth is the determined from the average of three locations in each forest. The soil moisture is measured as the volumetric water content (%). The data also has the daily rainfall in mm.THIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV

Daily soil moisture response to rainfall - Natural Forest and Exotic Pines

This data shows the daily soil moisture response to rainfall at a Natural Secondary Forest and Exotic Pine Planation in Trinidad and Tobago. The soil moisture is measured at 10, 20, 40 and 80 cm below the surface in each forest. The soil moisture at each depth is the determined from the average of three locations in each forest. The soil moisture is measured as the volumetric water content (%). The data also has the daily rainfall in mm.THIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV