Late Quaternary biotic and abiotic controls on long-term sediment flux in a northern Australian tropical river system

The modern distribution of monsoonal rainforest in the Australian tropics is patchy and is mainly associated with river corridors and groundwater springs, which indicates a strong dependence on hydrologic and geomorphic conditions. While their present distribution is well known, very little data exists on past spatial and temporal dynamics of these ecosystems, or their medium- to longer-term controls. Factors such as (i) fire frequency and type, and/or (ii) hydroclimatic conditions (e.g. droughts) have been proposed to control riverine corridor rainforest extent. Recent observations, however, also suggest an additional (iii) geomorphic control induced by alluvial knickpoint migration. Sediment sequences provide valuable archives for the reconstruction of longer-term (a) floodplain sedimentary dynamics, (b) local vegetation history, and (c) catchment-wide fire histories. This study investigates such a sediment sequence at Wangi Creek, and shows that a phase of aggradation, lasting ~4000 years, was recently disrupted by channel incision and floodplain erosion. The aggradational phase is characterized by sand deposition with average vertical floodplain accretion rates of 0.8 cm/yr and includes phases of soil development. The recent incisional phase has changed hydro-geomorphic conditions and caused widespread degradation of vegetation, erosion and lowering of the macro-channel surface. While there is no evidence in our data for an erosional event of similar magnitude since the onset of late Holocene floodplain aggradation, Wangi Creek experienced significant erosion and incision immediately before ~4000 years, providing the first evidence for a tropical cut-and-fill river system. We hence argue that phases of aggradation mainly controlled by biotic processes alternate and depend on feedbacks with incision phases controlled mainly by abiotic processes. The results show that eco-hydro-geomorphic feedbacks may play a crucial role in the medium- to longer-term history of tropical fluvial systems and need to be considered when interpreting fluvial archives with regards to climate, fire or human induced change. © 2019 John Wiley & Sons, Ltd.     

Precipitation in tropical America and the associated sources of moisture: a short review

Abstract

Understanding of the relationship between precipitation and the associated sources of moisture is essential to the improvement of our comprehension of the global water cycle. The observation of precipitation is one of the major challenges in the study of climate, as is the proper assignment of the sources of moisture that account for that precipitation. A stark contrast in the amounts of available information on precipitation may be seen in the cases of Central America and the northern part of South America. The main areas of precipitation in tropical America are described, and the moisture sources for these areas are identified by means of a Lagrangian approach presented with an example application. A strong relationship exists between the identified sources of moisture and the distribution of precipitation in the locations in question. The Caribbean Sea and the tropical Atlantic are highlighted as the main sources of moisture for the regions of highest precipitation in tropical America. Regional low-level winds play a major role in transport of moisture from the adjacent oceanic regions.

Editor Z.W. Kundzewicz

Citation Durán-Quesada, A.M., Reboita, M. and Gimeno, L., 2012. Precipitation in tropical America and the associated sources of moisture: a short review. Hydrological Sciences Journal, 57 (4), 612–624.     

Lithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto Rico

An extensive survey and topographic analysis of five watersheds draining the Luquillo Mountains in north‐eastern Puerto Rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS‐based topographic analysis was used to examine channel profiles, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial‐alluvial transitions. Non‐fluvial processes, such as landslides, deliver coarse boulder‐sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fining in the downstream reaches; a pattern associated with a mid‐basin transition between colluvial and fluvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid‐basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self‐forming ‘threshold’ alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach‐scale channel morphology, strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin‐scale patterns. Copyright © 2010 John Wiley and Sons, Ltd.     

Effect of land cover and hydro‐meteorological controls on soil water DOC concentrations in a high‐elevation tropical environment

Páramo soils store high amounts of organic carbon. However, the effects of climate change and changes in land cover and use (LC/LU) in this high‐elevation tropical ecosystem may cause a decrease in their carbon storage capacity. Therefore, better understanding of the factors influencing the Páramo soils' carbon storage and export is urgently needed. To fill this knowledge gap, we investigated the differences in dissolved organic carbon (DOC) content in the soil water of four LC/LU types (tussock grass, natural forest, pine plantations, and pasture) and the factors controlling its variability in the Quinuas Ecohydrological Observatory in south Ecuador. Weekly measurements of soil water DOC concentrations, meteorological variables, soil water content, and temperature from various depths and slope positions were monitored within the soils' organic and mineral horizons between October 2014 and January 2017. These data were used to generate regression trees and random forest statistical models to identify the factors controlling soil water DOC concentrations. From high to low concentrations, natural forest depict the highest DOC concentrations followed by pasture, tussock grass, and pine forest. For all LC/LU types, DOC concentrations increase with decreasing soil moisture. Our results also show that LC/LU is the most important predictor of soil water DOC concentrations, followed by sampling depth and soil moisture. Interestingly, atmospheric variables and antecedent evapotranspiration and precipitation conditions show only little influence on DOC concentrations during the monitoring period. Our findings provide unique information that can help improve the management of soil and water resources in the Páramo and other peat dominated ecosystems elsewhere.     

Water transport and tracer mixing in volcanic ash soils at a tropical hillslope: A wet layered sloping sponge

Andosol soils formed in volcanic ash provide key hydrological services in montane environments. To unravel the subsurface water transport and tracer mixing in these soils we conducted a detailed characterization of soil properties and analyzed a 3-year data set of sub-hourly hydrometric and weekly stable isotope data collected at three locations along a steep hillslope. A weakly developed (52–61 cm depth), highly organic andic (Ah) horizon overlaying a mineral (C) horizon was identified, both showing relatively similar properties and subsurface flow dynamics along the hillslope. Soil moisture observations in the Ah horizon showed a fast responding (few hours) “rooted” layer to a depth of 15 cm, overlying a “perched” layer that remained near saturated year-round. The formation of the latter results from the high organic matter (33–42%) and clay (29–31%) content of the Ah horizon and an abrupt hydraulic conductivity reduction in this layer with respect to the rooted layer above. Isotopic signatures revealed that water resides within this soil horizon for short periods, both at the rooted (2 weeks) and perched (4 weeks) layer. A fast soil moisture reaction during rainfall events was also observed in the C horizon, with response times similar to those in the rooted layer. These results indicate that despite the perched layer, which helps sustain the water storage of the soil, a fast vertical mobilization of water through the entire soil profile occurs during rainfall events. The latter being the result of the fast transmissivity of hydraulic potentials through the porous matrix of the Andosols, as evidenced by the exponential shape of the water retention curves of the subsequent horizons. These findings demonstrate that the hydrological behavior of volcanic ash soils resembles that of a “layered sponge,” in which vertical flow paths dominate.     

Estimation of evapotranspiration and its components from an apple orchard in northwest China using sap flow and water balance methods

Field experiments were conducted to investigate the effects of leaf area index and soil moisture content on evapotranspiration and its components within an apple orchard in northwest China for 2 years. Evapotranspiration in the non‐rainfall period was estimated using two approaches: the soil water balance method based on tube‐type time‐domain reflection measurements, and sap flow plus micro‐lysimeter methods. The two methods were in good agreement, with differences usually less than 10%. The components of evapotranspiration varied with canopy development. During spring and autumn, soil evaporation was dominating as result of low leaf area index. In summer, plant transpiration became significant, with an average transpiration to evapotranspiration ratio of 0·87. The crop coefficient K_c showed a strong linear dependence on leaf area index. The water stress coefficient K_s was around 1·0 when soil moisture was above 23% and started to decrease linearly after that. This study demonstrates that prediction of evapotranspiration in apple orchards can be made using the Food and Agriculture Organization's crop coefficient method from commonly available meteorological data in the area. Copyright © 2006 John Wiley & Sons, Ltd.     

Estimating water balance components in irrigated agriculture using a combined approach of soil moisture and energy balance monitoring,and numerical modelling

Information on water balance components such as evapotranspiration and groundwater recharge are crucial for water management. Due to differences in physical conditions, but also due to limited budgets, there is not one universal best practice, but a wide range of different methods with specific advantages and disadvantages. In this study, we propose an approach to quantify actual evapotranspiration, groundwater recharge and water inflow, i.e. precipitation and irrigation, that considers the specific conditions of irrigated agriculture in warm, arid environments. This approach does not require direct measurements of precipitation or irrigation quantities and is therefore suitable for sites with an uncertain data basis. For this purpose, we combine soil moisture and energy balance monitoring, remote sensing data analysis and numerical modelling using Hydrus. Energy balance data and routine weather data serve to estimate ET₀. Surface reflectance data from satellite images (Sentinel-2) are used to derive leaf area indices, which help to partition ET₀ into energy limited evaporation and transpiration. Subsequently, first approximations of water inflow are derived based on observed soil moisture changes. These inflow estimates are used in a series of forward simulations that produce initial estimates of drainage and ET_act, which in turn help improve the estimate of water inflow. Finally, the improved inflow estimates are incorporated into the model and then a parameter optimization is performed using the observed soil moisture as the reference figure. Forward simulations with calibrated soil parameters result in final estimates for ET_act and groundwater recharge. The presented method is applied to an agricultural test site with a crop rotation of cotton and wheat in Punjab, Pakistan. The final model results, with an RMSE of 2.2% in volumetric water content, suggest a cumulative ET_act and groundwater recharge of 769 and 297 mm over a period of 281 days, respectively. The total estimated water inflow accounts for 946 mm, of which 77% originates from irrigation.