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"Subsurface drainage Tropics."
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Surface and subsurface runoff generation processes in a poorly gauged tropical coastal catchment : a study from Nicaragua : dissertation
Hydrological research in humid tropics is particularly challenging because of highly variable hydrological conditions and high socio-economic stresses caused by rapid population increase, as is the case of Nicaragua. The objective of this research is to understand the surface and subsurface runoff generation processes in a poorly gauged coastal catchment in Nicaragua under variable humid tropical conditions. Specifically, it focuses on identifying geomorphological and hydro-climatic controls on catchment response at different spatio-temporal scales and studies the link between hydrological processes and ecosystem conditions.
Book
Streamflow generation in a nested system of intermittent and perennial tropical streams under changing land use
Despite the increased interest in the hydrology of intermittent hydrological streams in recent years, little attention has been given to these systems in tropical forest environments. We present a unique set of hydrometric, stable isotopic, geochemical, and landscape mapping information to obtain a mechanistic understanding of streamflow generation in 20 nested catchments ( 1-159 km.sup.2) draining intermittent and perennial streams and rivers in the Chocó-Darien ecoregion, a tropical biodiversity hotspot, located in the Pacific lowlands of northern Ecuador that has been strongly degraded by deforestation and agricultural encroachment during the last half-century. Hydrological intermittency is mainly controlled by antecedent wetness due to the strong seasonality of precipitation. Nevertheless, the streambed of catchments draining intermittent streams remains humid throughout the year, even when surface water stops flowing, since evapotranspiration is reduced due to continued cloudy and foggy conditions during the dry season. Intermittent streams mainly located in conserved forested headwaters with shallow soils and a low permeability bedrock have a faster streamflow response to rainfall and shorter recession times than the perennial streams with high permeability bedrock in the catchment's degraded middle and lower parts. Isotopic information shows that rainfall during the wet period (January to May) contributes to streamflow generation in the intermittent streams, whereas rainfall during the wet season recharges the subsurface water storage of the perennial streams. Concentrations of major ions and electrical conductivity were lower in intermittent streams compared to perennial streams. We found a strong correlation between the catchments' geology and geochemical signals and a weak correlation with the topography, land cover, and soil type. These findings indicate that shallow subsurface flow paths through the organic horizon of the soil dominate streamflow generation in intermittent streams due to the limited water storage capacity of their bedrock with very low permeability. On the contrary, high bedrock permeability increases the water storage capacity and is replenished during the wet period, helping sustain streamflow generation throughout the year for the perennial streams. These findings suggest that geology may play an important role in driving hydrological intermittency, even in highly degraded tropical forest catchments, and provide key process-based information useful for water management and hydrological modelling of intermittent hydrological systems.
Journal Article
Fire Aerosols Slow Down the Global Water Cycle
Fire is an important Earth system process and the largest source of global primary carbonaceous aerosols. Earlier studies have focused on the influence of fire aerosols on radiation, surface climate, air quality, and biogeochemical cycle. The impact of fire aerosols on the global water cycle has not been quantified and related mechanisms remain largely unclear. This study provides the first quantitative assessment and understanding of the influence of fire aerosols on the global water cycle. This is done by quantifying the difference between simulations with and without fire aerosols using the coupled Community Earth System Model (CESM). Results show that present-day fire aerosols weaken the global water cycle significantly. They decrease the continental precipitation, evapotranspiration, and runoff by 4.1 ± 1.8, 2.5 ± 0.5, and 1.5 ± 1.4 x 10³ km³ yr−1 as well as ocean evaporation, precipitation, and water vapor transport from ocean to land by 8.1 ± 1.9, 6.6 ± 2.3, and 1.5 ± 1.4 x 10³ km³ yr−1. The impacts of fire aerosols are most clearly seen in the tropics and the Arctic-boreal zone. Fire aerosols affect the global water cycle mainly by cooling the surface, which reduces ocean evaporation, land soil evaporation, and plant transpiration. The decreased water vapor load in the atmosphere leads to a decrease in precipitation, which contributes to reduced surface runoff and subsurface drainage.
Journal Article
Wastewater treatment in tsunami affected areas of Thailand by constructed wetlands
The tsunami of December 2004 destroyed infrastructure in many coastal areas in South-East Asia. In January 2005, the Danish Government gave a tsunami relief grant to Thailand to re-establish the wastewater management services in some of the areas affected by the tsunami. This paper describes the systems which have been built at three locations: (a) Baan Pru Teau: A newly-built township for tsunami victims which was constructed with the contribution of the Thai Red Cross. Conventional septic tanks were installed for the treatment of blackwater from each household and its effluent and grey water (40 m3/day) are collected and treated at a 220 m2 subsurface flow constructed wetland. (b) Koh Phi Phi Don island: A wastewater collection system for the main business and hotel area of the island, a pumping station and a pressure pipe to the treatment facility, a multi-stage constructed wetland system and a system for reuse of treated wastewater. The constructed wetland system (capacity 400 m3/day) consists of vertical flow, horizontal subsurface flow, free water surface flow and pond units. Because the treatment plant is surrounded by resorts, restaurants and shops, the constructed wetland systems are designed with terrains as scenic landscaping. (c) Patong: A 5,000 m2 constructed wetland system has been established to treat polluted water from drainage canals which collect overflow from septic tanks and grey water from residential areas. It is envisaged that these three systems will serve as prototype demonstration systems for appropriate wastewater management in Thailand and other tropical countries.
Journal Article
The removal of nutrients from plant nursery irrigation runoff in subsurface horizontal-flow wetlands
In New South Wales (NSW) Australia, the recent introduction of legislation to control runoff and charge for water used in agricultural production has encouraged commercial plant nurseries to collect and recycle their irrigation drainage. Runoff from a nursery typically contains around 6 mg/L TN (> 70% as NO3), 0.5 mg/L TP (> 50% as PO4), and virtually no organic matter (BOD <5 mg/L; DOC <20 mg/L). As a result, algal blooms frequently occur in storage dams. This paper describes a study evaluating the effectiveness of subsurface flow wetlands in the removal of nutrients from nursery runoff on the sub-tropical northern coast of NSW, Australia. Four experimental subsurface flow wetlands (1 m×4 m×0.5 m water depth) were planted with Phragmites australis in April 1999. TN and TP load removals were > 84% and > 65% respectively at HRTs of between 5 and 2 days, with the majority of out-flowing TN and TP being organic in form. Internal generation of organic N and P resulted in persistent background levels of 0.45 mg/L TN and 0.15 mg/L TP in the reed bed effluent. TN, NH4 and TP removal was affected by HRT (P <0.05). Greater than 90% load removal of NH4, NO2, NO3 and Ortho-P was achieved at all HRTs, with outlet concentrations generally <0.01 mg/L for all. For TN, a strong relationship existed between removal rate (g/m2/day) and loading rate (r2=0.995), while a weaker relationship existed for TP (r2=0.47). It is estimated that a 1 ha nursery would require a reed bed area of 200 m2 for a 2 day HRT.
Journal Article
The filter system for tertiary treatment of sewage effluent by land application - its performance in a subtropical environment
FILTER is an innovative, CSIRO developed system for treating effluent using high rate land application and subsequent effluent recapture via a closely spaced, subsurface drainage network. We report on the summer performance of a FILTER system established in a subtropical environment on a relatively impermeable swelling clay soil underlain by a deep regional water table. Using secondary treated sewage effluent, the FILTER system produced effluent of tertiary nutrient standards (≤5 mg/L TN; ≤1 mg/L TP), with salinity levels suitable for subsequent irrigation reuse (EC ≤2.5 dS/m). Removal of faecal coliforms was considerably less effective. The hydraulic loading rate achieved was about two and a half times larger than conventional irrigation demand, but this was associated with high deep percolation losses (c 3 mm/day). Comparisons are made with the original FILTER system developed and tested by Jayawardane et al. in temperate Australia. Suggestions are made for modifications to, and further testing of FILTER in a subtropical environment.
Journal Article
Observations of the transient characteristics of the hydrological balance
The impact of transient input events on the hydrological balance components are studied over a wide range of spatial and temporal scales. First, interception evaporation and energy-balance changes during and following precipitation events are examined at the site spatial and event temporal scale. Second, the hydrological response to spatial and seasonal changes in precipitation and evapotranspiration due to land cover changes are examined on the small-to-large watershed scale, covering the event to seasonal time scales. Estimates in the literature for interception evaporation using conventional methods vary widely even over the same forest type. We present an alternative method to estimate interception evaporation from eddy covariance measurements combined with a novel use of data-analysis techniques to form base state and event-based ensembles. Application of this method at a tropical rain forest site in the Amazon resulted in mean interception evaporation estimates of 11.6%, comparable to recent conventional studies in that region. Energy balance comparisons between dry and afternoon rain-days show an approximately 15% increase of evaporative fraction on the rain days, with the energy being supplied by a corresponding decrease in the canopy heat storage. Large differences in the watershed response characteristics of streamflow peak and streamflow peak to watershed precipitation ratio in the Catskill-Hudson Valley Region of New York were observed relative to a watershed’s proximity to a precipitation shadow observed there. There were detectable changes in streamflow and soil moisture recession times, as well as in the diurnal streamflow amplitude over this watershed network during the autumn transition. This provides an independent estimate of autumn or spring onset to complement phenological or satellite-derived measurements, and is important as this shows the timing of the hydrological impacts of abrupt changes in evapotranspiration forcing on the watersheds. In a network of watersheds in the Amazon, precipitation and subsequent storage and subsurface drainage processes seem to have a greater influence on seasonal changes in soil moisture recession compared with seasonal variations in evapotranspiration due to vegetation state.
Dissertation
