Tropical Hydrology Research Articles

Preface special issue: “Processes and patterns in tropical hydrology”

Preface special issue: “Processes and patterns in tropical hydrology”

Revisiting Michael Bonell's work on humid tropical rainforest catchments: Isotope tracers reveal seasonal shifts in catchment hydrology

It has been almost 50 years since the foundational work at the Babinda catchments in North Queensland kickstarted the field of tropical hydrology globally. To expand upon this work and build a more generalized hydrological understanding of steep rainforest catchments, we studied the seasonal evolution of hydrological response from two catchments with broadly similar characteristics to the Babinda catchments. Both hydrometric and water stable isotope data were collected at relatively high frequencies during one wet season (Thompson Creek) and a 3-year period (Atika Creek). The longer dataset spans a wide range of environmental conditions experienced in the humid tropics, including events that cover the wetting-up transitional period of the wet season and tropical cyclones (TC). Both catchments displayed a fast streamflow response to rainfall with the shallow upper soil profile responding quickly to rainfall at Atika Creek. New findings from this study include the importance of pre-event water (>50% using the two component hydrograph separation technique) for overall event flows, especially when the catchment was wet. Rainfall, surface runoff and groundwater isotope and specific electrical conductivity (SEC) compositions varied between rainfall events with the most complex bivariate mixing plots observed for multi-peak events that occurred at the start of the wet season and after a dry period within the wet season. Two-tracer, 3 component hydrograph separations did not provide satisfactory results in identifying source water contributions to streamflow. These results highlighted the time-variant and non-conservative behaviour of the rainfall, surface runoff and shallow groundwater source waters over the seasonal timescale, with soil water being an important unidentified source contributor. Our findings highlight the need for high frequency multi-source sampling to accurately interpret catchment behaviour and the importance of soil water contributions to streamflow. We propose a framework to describe the seasonal evolution of streamflow response in steep tropical rainforest catchments experiencing seasonal rainfall activity.

Rainfall Runoff Balance Enhanced Model Applied to Tropical Hydrology

The integrative and comprehensive analysis considering the spatial and temporal representation of the hydrological process, such as the distribution of rainfall, land cover and land use, is a challenge for the water resources management. In tropical areas, energy availability throughout the year defines the rainfall distribution and evapotranspiration rate according to vegetation heterogeneity. To quantify water balance in tropical areas including these heterogeneities in the soil-vegetation-atmosphere relationship, we developed a fully distributed hydrological model called the Rainfall Runoff Balance Enhanced Model (RUBEM). The model was developed under a physics-based process structure, using remote sensing data to represent soil-water balance patterns, such as evapotranspiration, interception, baseflow, lateral flow, recharge, and runoff. The calibration procedure was based on nine global parameters. RUBEM could represent the spatio-temporal heterogeneities (soil, land use and land cover (LULC), topography, vegetation, and climate) in three basins in a tropical area. The results showed good adherence between the processes governing the soil-vegetation-atmosphere relationship according to the humidity indicator and the runoff coefficient. Overall, RUBEM can be used to help improve the management and planning of integrated water resources under climate, land use, and land cover changes in tropical regions.

The pantropical response of soil moisture to El Niño

Abstract. The 2015–2016 El Niño event ranks as one of the most severe on record in terms of the magnitude and extent of sea surface temperature (SST) anomalies generated in the tropical Pacific Ocean. Corresponding global impacts on the climate were expected to rival, or even surpass, those of the 1997–1998 severe El Niño event, which had SST anomalies that were similar in size. However, the 2015–2016 event failed to meet expectations for hydrologic change in many areas, including those expected to receive well above normal precipitation. To better understand how climate anomalies during an El Niño event impact soil moisture, we investigate changes in soil moisture in the humid tropics (between ±25∘) during the three most recent super El Niño events of 1982–1983, 1997–1998 and 2015–2016, using data from the Global Land Data Assimilation System (GLDAS). First, we use in situ soil moisture observations obtained from 16 sites across five continents to validate and bias-correct estimates from GLDAS (r2=0.54). Next, we apply a k-means cluster analysis to the soil moisture estimates during the El Niño mature phase, resulting in four groups of clustered data. The strongest and most consistent decreases in soil moisture occur in the Amazon basin and maritime southeastern Asia, while the most consistent increases occur over eastern Africa. In addition, we compare changes in soil moisture to both precipitation and evapotranspiration, which showed a lack of agreement in the direction of change between these variables and soil moisture most prominently in the southern Amazon basin, the Sahel and mainland southeastern Asia. Our results can be used to improve estimates of spatiotemporal differences in El Niño impacts on soil moisture in tropical hydrology and ecosystem models at multiple scales.

A province with several climates: What knowledge of hydrology can be useful? A literature review

This article explores the hydrological state of the art that could be useful for a climate-diverse region, such as Manabi province in Ecuador. The province, with an area of 18,940 km2, has a variety of tropical and dry climates. First, a review of the climatic conditions of the study area is carried out, since different sources disagree to some extent regarding climate types; then, a critical assessment of the relevant hydrologic literature is carried out. Here it is found that the literature best-suited for the conditions of Manabi includes literature for semi-arid, savannah and humid tropical hydrology. Several research gaps are found, but also a significant body of knowledge that could serve as a base for future studies. It should be noted that much of the available literature was obtained with vegetation and soil conditions that might differ from those of Manabi. Therefore, it is essential that findings from abroad are contrasted with results obtained in Manabi. Understanding of the hydrological functioning of the study area could contribute to the hydrologic literature for semi-arid, savannah and humid tropic climates.

Contribución del Bosque en Cerro Pelado a la Mitigación del Cambio Climático: Almacenamiento y Secuestro de Carbono

This study deals with the estimation of the carbon associated to the Above Ground Biomass, in a permanent plot of measurement located in the Cerro Pelado Tropical Hydrology Observatory in Gamboa, Panama. The objective is to highlight the contribution made by one-hectare of this forest to the local Climate Change mitigation. We employed a relation to carbon storage and sequestration, based on the important species of the tree community with diameter at breast height equal to or greater than 10 cm. It was found an increase in the Above Ground Biomass at the plot, with values of: 193 Mg*ha-1, 218 Mg*ha-11 and 225 Mg*ha-1, for the 2008, 2012 and 2015 censuses, respectively. The net growth in Above Ground Biomass is eliminating approximately 7 to 8 Mg de CO2*ha-1year-1 from the atmosphere, which means that it is acting as a sink and sequestrator of CO2. In this habitat, it is recognizable the important contribution of the Pera arborea species in terms of sustaining carbon sequestration in the Above Ground Biomass.Keywords: Gamboa, Panama, Above Ground Biomass, Climate Change, CO2

Large deglacial shifts of the Pacific Intertropical Convergence Zone

The position of the Intertropical Convergence Zone (ITCZ) is sensitive to changes in the balance of heat between the hemispheres which has fundamental implications for tropical hydrology and atmospheric circulation. Although the ITCZ is thought to experience the largest shifts in position during deglacial stadial events, the magnitude of shifts has proven difficult to reconstruct, in part because of a paucity of high-resolution records, particularly those including spatial components. Here we track the position of the ITCZ from 150 to 110 ka at three sites in the central equatorial Pacific at sub-millennial time resolution. Our results provide evidence of large, abrupt changes in tropical climate during the penultimate deglaciation, coincident with North Atlantic Heinrich Stadial 11 (∼136–129 ka). We identify this event both as a Northern Hemisphere increase in aeolian dust and as a shift in the mean position of the ITCZ a minimum of 4° southwards at 160° W.

Composition, Diversity, and Tree Structure of a Tropical Moist Forest in Gamboa, Colon, Panama

This study focuses on the floristic diversity of the forest trees found at Cerro Pelado Tropical Hydrology Observatory in Gamboa, Colon, Panama. Field work for the quantitative inventory was carried out in November 2012. Data were collected by assessing the status of the forest in terms of tree species diversity and structure from one-hectare plot divided into twenty-five 20 m × 20 m quadrats. All tree species were identified, and their diameters at breast height (dbh) were measured. A total of 384 individuals with (dbh ≥ 10 cm) were counted, corresponding to 28 families, 41 genera, and 43 species of which Pera arborea, Oenocarpus mapora, Amaioua corymbosa, Vantanea depleta, and Matayba apetala were the species with the highest ecological weight, achieving 57.99% of the importance value index. Results were compared with plots from other forests of the Panama Canal watershed in terms of diversity and number of species per area and number of individuals. It was found that the habitat studied has low diversity, with very few species in the study area, even though some of them were very abundant.

Northeast African temperature variability since the Late Pleistocene

Abstract The development and application of lacustrine paleotemperature proxies based on microbial membrane lipid structures, including the TEX86 and branched glycerol dialkyl glycerol tetraether (brGDGT) paleothermometers, have greatly advanced our understanding of the late-glacial and postglacial temperature history of Africa. However, the currently available records are from equatorial and southern hemisphere sites, limiting our understanding of the spatial patterns of temperature change. Here we use the brGDGT paleotemperature proxy to reconstruct Late Pleistocene and Holocene temperatures from Lake Tana, Ethiopia (12°N, 37°E). Following the termination of Heinrich Stadial 1 at ~ 15 ka, Lake Tana experienced a 3.7 °C oscillation over 1.2 ky. Temperatures then increased abruptly by nearly 7 °C between 13.8 and 13.0 ka, followed by a slow warming trend that peaked during the mid Holocene. Temperatures subsequently cooled from ~ 6 ka to ~ 0.4 ka. These data indicate that temperature at Lake Tana was sensitive to climate changes caused by variations in the Atlantic Meridional Overturning Circulation during the Late Pleistocene, as well as to regional hydroclimatic changes and reorganizations of the monsoons. Our record suggests that late-glacial temperature changes in northeast Africa were linked to high-latitude northern hemispheric climate processes, but that subsequent post-glacial temperature variations were strongly influenced by tropical hydrology.

A 0.6 million year record of millennial‐scale climate variability in the tropics

AbstractA ~600 kyr long scanning X‐ray fluorescence record of redox variability from the Cariaco Basin, Venezuela, provides insight into rapid climate change in the tropics over the past five glacial‐interglacial cycles. Variations in the sediment accumulation of the redox‐sensitive element molybdenum (Mo) can be linked to changes in Intertropical Convergence Zone migration and reveal that millennial‐scale variability is a persistent feature of tropical climate over the past 600 kyr, including during periods of interglacial warmth. This new record supports the idea that high‐frequency tropical climate variability is not controlled solely by ice volume changes, with implications for the role of high‐latitude forcing of Intertropical Convergence Zone position and tropical hydrology on millennial timescales.

Low latitude hydro-climatic changes during the Plio-Pleistocene: evidence from high resolution alkane records in the southern South China Sea

High resolution records of long chain n-alkane biomarkers from the southern South China Sea reveal tightly response of n-alkane distribution to hydro-climate changes over the past 5 Ma, with increasing longer chain n-C31 alkanes indicating a correlation with drier conditions and increasing shorter chain n-C27 alkanes with wetter conditions. The variations of the C31/C27 ratio, or the alkane chain length index, imply humid conditions before 2.9 Ma, progressively reduced moisture since then and to bigger fluctuations between wet and dry conditions since 1.2 Ma. This long term hydro-climate trend is superimposed by glacial dry and interglacial wet patterns over the Plio-Pleistocene glacial cycles. Combined with other proxy records, our results indicate that precipitation over the tropical Asia-Pacific strengthened before the onset of the northern hemisphere glaciation and the mid-Pleistocene climate transition at about 1.2 Ma. These dramatic humidity changes over major climate transitions imply a crucial role of tropical hydrology dynamics on global climate change in the late Cenozoic.

The Precipitation Characteristics of ISCCP Tropical Weather States

Abstract The authors examine the daytime precipitation characteristics of the International Satellite Cloud Climatology Project (ISCCP) weather states in the extended tropics (35°S–35°N) for a 10-yr period. The main precipitation dataset used is the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis operational product 3B42 dataset, but Global Precipitation Climatology Project daily data are also used for comparison. It is found that the most convectively active ISCCP weather state (WS1), despite an occurrence frequency below 10%, is the most dominant state with regard to surface precipitation, producing both the largest mean precipitation rates when present and the largest percent contribution to the total precipitation of the tropics; yet, even this weather state appears to not precipitate about half the time, although this may be to some extent an artifact of detection and spatiotemporal matching limitations of the precipitation dataset. WS1 exhibits a modest annual cycle of the domain-average precipitation rate, but notable seasonal shifts in its geographic distribution. The precipitation rates of the other weather states appear to be stronger when occurring before or after WS1. The precipitation rates of the various weather states are different between ocean and land, with WS1 producing higher daytime rates on average over ocean than land, likely because of the larger size and more persistent nature of oceanic WS1s. The results of this study, in addition to advancing the understanding of tropical hydrology, can serve as higher-order diagnostics for evaluating the realism of tropical precipitation distributions in large-scale models.

Speleothem records of changes in tropical hydrology over the Holocene and possible implications for atmospheric methane

Recent speleothem records from the tropics of both hemispheres document a gradual decrease in the intensity of the monsoons in the Northern Hemisphere and increase in the Southern Hemisphere monsoons over the Holocene. These changes are a direct response of the monsoons to precession-driven insolation variability. With regard to atmospheric methane, this shift should result in a decrease in Northern Hemisphere tropical methane emissions and increase in Southern Hemisphere emissions. It is plausible that that overall tropical methane production experienced a minimum in the mid-Holocene because of decreased seasonality in rainfall at the margins of the tropics. Changes in tropical methane production alone might, therefore, explain many of the characteristics of Holocene methane concentrations and isotopic chemistry.

First-order catchment mass balance during the wet season in the Panama Canal Watershed

Summary Tropical hydrology is poorly understood for a number of reasons. Intense biological activity in the tropics introduces complexities to the hydrologic process. Bioturbation, rapid rates of decay, and intensive insect activity all tend to promote rapid flow paths in the upper soil. Aggressive weathering leads to clays depleted of light cations and deep soil profiles. Processes in the seasonal tropics are further complicated by seasonal transitions, and very large changes in catchment storage between seasons. Beginning in 2005, we installed a suite of hydrologic sensors in a 16.7 ha first-order catchment in the Panama Canal Watershed to observe hydrologic variables and identify the dominant streamflow generation processes. The site is located near the village of Gamboa, which is located on the east bank of the Panama Canal at the confluence of Lake Gatun and the Chagres River. The study catchment is located on the north side of a ridge off the eastern flank of a 230 m tall hill known as Cerro Pelado, and is covered by 70–120 year old re-growth triple-canopy forest. Measurements included: rainfall above the canopy, throughfall, stemflow, evapotranspiration, shallow groundwater levels and streamflow. Deep groundwater storage was not measured. This paper describes measurements made, data collected, and the worth of those data in estimating the mass balance closure of a first-order catchment during the wet season. We compare measurements of the different components of the water cycle with observations from other published studies from the tropics. Data analysis results indicate water balance closure errors of approximately 8%.

Development of the forSIM model to quantify positive and negative hydrological impacts of tropical reforestation

Existing approaches to modelling the impacts of reforestation on tropical hydrology only simulate one or two changes, thereby limiting our ability to quantify the balance between complex positive and negative changes, even for a single micro-basin. To initiate a more holistic and multi-scale approach, we develop a new simulation model structure within the Matlab-Simulink systems environment that firstly, illustrates quantifiable interrelationships between reforestation-related hydrological changes in the component systems of evapotranspiration, runoff generation, sediment delivery and nutrient processes. Secondly, the model structure allows us to highlight basin-scale time-series observations needed to quantify reforestation-related changes in the component hydrological processes. The dynamic model developed is called forSIM and comprises of component models that are derived by means of the data-based mechanistic (DBM) philosophy. Such a modelling approach is required to constrain the large uncertainties that can arise from whole system modelling. The review of the hydrological processes and controlling characteristics that change following reforestation and, therefore, need to be simulated, has highlighted the lack of basin-scale time-series observations of the potentially positive impacts of ‘protective’ reforestation on sediment and nutrient delivery, and the need to utilise more macro-scale data. The next phase of the modelling process is to derive estimates of systems parameters and simulation scenarios for specific macro-basins in the tropics undergoing extensive reforestation.

Seawater isotope constraints on tropical hydrology during the Holocene

Paleoceanographic data from the low latitude Pacific Ocean provides evidence of changes in the freshwater budget and redistribution of freshwater within the basin during the Holocene. Reconstructed Holocene seawater δ18O changes compare favorably to differences predicted between climate simulations for the middle Holocene (MH) and for the pre‐Industrial late Holocene (LH). The model simulations demonstrate that changes in the tropical hydrologic cycle affect the relationship between δ18Osw and surface salinity, and allow, for the first time, quantitative estimates of western Pacific salinity change during the Holocene. The simulations suggest that during the MH, the mean salinity of the Pacific was higher because less water vapor was transported from the Atlantic Ocean and more was transported to the Indian Ocean. The salinity of the western Pacific was enhanced further due both to the greater advection of salt to the region by ocean currents and to an increase in continental precipitation at the expense of maritime precipitation, the latter a consequence of the stronger Asian summer monsoon.

Solar and anthropogenic forcing of tropical hydrology

Holocene climate proxies suggest substantial correlations between tropical meteorology and solar variations, but these have thus far not been explained. Using a coupled ocean‐atmosphere‐composition model forced by sustained multi‐decadal irradiance increases, we show that greater tropical temperatures alter the hydrologic cycle, enhancing the climatological precipitation maxima in the tropics while drying the subtropical subsidence regions. The shift is enhanced by tropopause region ozone increases, and the model captures the pattern inferred from paleoclimate records. The physical process we describe likely affected past civilizations, including the Maya, Moche, and Ancestral Puebloans who experienced drought coincident with increased irradiance during the late medieval (∼900–1250). Similarly, decreased irradiance may have affected cultures via a weakened monsoon during the Little Ice Age (∼1400–1750). Projections of 21st‐century climate change yield hydrologic cycle changes via similar processes, suggesting a strong likelihood of increased subtropical drought as climate warms.

Wet phases in tropical southern Africa during the last glacial period

A 45,000‐year high‐resolution sedimentary record from Lake Masoko in Tanzania shows the climate in this part of East Africa to have been characterized by a short and less severe dry season during the Last Glacial Maximum. Moisture and lake‐level proxies, pollen assemblages and magnetic susceptibility, indicate that rainfall in the Masoko area was strongly controlled by low‐latitude insolation (i.e., precessional forcing). Such observations contrast with the climatic patterns previously reconstructed in East Africa for the last glacial. Indeed, widespread dry conditions are generally observed and are attributed to the predominant control of high latitude glacial forcing over insolation forcing on the tropical hydrology. However, during the Younger Dryas cold event, out of phase with regional insolation behaviour, wetter conditions prevailed in this area, suggesting that the rainfall belt over Africa was probably shifted to the South, bringing moisture to the southernmost tropics.

Modern stable isotopic (δ18O, δ2H, δ13C) variation in terrestrial, fluvial, estuarine and marine waters from north‐central Sarawak, Malaysian Borneo

AbstractStable isotope data on humid tropical hydrology are scarce and, at present, no such data exist for Borneo. Delta18O, δ2H and δ13C were analysed on 22 water samples from different parts of the Sungai (river) Niah basin (rain, cave drip, rainforest pool, tributary stream, river, estuary, sea) in north‐central Sarawak, Malaysian Borneo. This was done to improve understanding of the modern stable isotope systematics of the Sungai Niah basin, essential for the palaeoenvironmental interpretation of the Late Quaternary stable isotope proxies preserved in the Great Cave of Niah. The Niah hydrology data are put into a regional context using the meteoric water line for Southeast Asia, as derived from International Atomic Energy Agency/World Meteorological Organization isotopes in precipitation network data. Although the Niah hydrological data‐set is relatively small, spatial isotopic variability was found for the different subenvironments of the Sungai Niah basin. A progressive enrichment occurs towards the South China Sea (δ18O −4·6‰; δ2H −29·3‰; δ13C −4·8‰) from the tributary stream (δ18O −8·4‰; δ2H −54·7‰; δ13C −14·5‰) to up‐river (δ18O c. −8‰; δ2H c. −51‰; δ13C c. −12‰) and down‐river values (δ18O c. −7·5‰; δ2H c. −45‰; δ13C c. −11‰). This is thought to reflect differential evaporation and mixing of different components of the water cycle and a combination of depleted biogenic δ13C (plant respiration and decay) with enriched δ13C values (due to photosynthesis, atmospheric exchange, mixing with limestone and marine waters) downstream. Cave drip waters are relatively enriched in δ13C as compared to the surface waters. This may indicate rapid degassing of the cave drips as they enter the cave atmosphere. Copyright © 2005 John Wiley & Sons, Ltd.

Positive Feedback and System Resilience from Graphical and Finite-Difference Models: The Amazon Ecosystem—An Example

Abstract Positive feedback has the potential to create multiple steady states that divide system space into regions of distinct behavioral modes. Graphical analysis can be used to recognize systems likely to behave in this way. For example, positive feedback created by interactions among tropical forest, regional hydrology, and climate in the Amazon basin may produce multiple steady states. As deforestation progresses, one state—the normal forest with a high leaf area index—is stable and promotes system resilience. However, deforestation can cause the system to shift toward and past a second, unstable state at moderate levels of forest cover. The latter separates forest stability in the face of stress from runaway behavior and system collapse into an alternative grassland ecology. To test this inference, a two-dimensional finite-difference model has been constructed that includes parameterizations of the functions controlling feedback. Results of the experiment indicate that human-driven deforestation may...