Open Water Surface Research Articles

Open surface water index: a novel approach for surface water mapping and extraction using multispectral and multisensory data

ABSTRACT In the present study, a novel water index has been proposed for mapping of open surface water in multispectral images and named as open surface water index (OSWI). The proposed index has been applied on Landsat-8 data to delineate the water area and the results are compared with those of existing water indices. The index is also applied on Sentinel-2A data and corresponding water pixels are extracted. The results have been validated with the help of classification as well as density slicing based methods on multisensory data. The preliminary results show that the proposed index is capable of identifying smaller water regions which remain unidentified by other indices.

Occurrence, distribution, and risk assessment of pharmerciuticals in wastewater and open surface drains of peri-urban areas: Case study of Juja town, Kenya

The occurrence of Active Pharmaceutical Ingredients (APIs) in the environment is becoming a major area of concern due to their undesirable effects on non-target organisms. This study investigated the occurrence and risk of contamination by five antibiotics and three antiretrovirals drugs in a fast-growing peri-urban area in Kenya, with inadequate sewer system coverage. Due to poor sewage connectivity and poorly designed decentralized systems, wastewater is directly released in open drains. Water and sediment samples were collected from open surface water drains, while wastewater samples were collected from centralized wastewater treatment plants (WWTP). Solid-phase extraction and ultrasonic-assisted extraction for the aqueous and sediment samples respectively were carried out and extracts analyzed by liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) using isotopically labeled internal standards. APIs were observed with the detection frequency ranging from 36% to 100%. High mean concentrations of 48.7 μg L−1, 108 μg L−1, and 532 μg L−1 were observed in surface drains for Lamivudine (3 TC), Sulfamethoxazole (SMX), Ciprofloxacin (CIP) respectively. Drain sediments also showed high concentrations of APIs ranging from 2.1 to 13,100 μg kg−1. APIs in this study exceeded those observed in existing literature studies. JKUAT WWTP removal efficiencies varied from −90.68% to 72.67%. Total APIs emission load of the study area was 3550 mg d−1 with WWTP effluent contributing higher loads (2620 mg d−1) than surface water drains (640 mg d−1). Zidovudine (ZDV), nevirapine (NVP), and trimethoprim (TMP) loads in drains, however, exceeded WWTP effluent. Low to high ecotoxicity risk of the individual APIs were observed to the aquatic environment, with high risks for the development of antibiotic resistance in microbiome as determined by the risk quotient (RQ) approach. Risk management through efficient wastewater collection, conveyance, and treatment is necessary to suppress the measured concentrations.

Denitrification and associated nitrous oxide and carbon dioxide emissions from the Amazonian wetlands

Abstract. In this paper, we quantify the CO2 and N2O emissions from denitrification over the Amazonian wetlands. The study concerns the entire Amazonian wetland ecosystem with a specific focus on three floodplain (FP) locations: the Branco FP, the Madeira FP and the FP alongside the Amazon River. We adapted a simple denitrification model to the case of tropical wetlands and forced it by open water surface extent products from the Soil Moisture and Ocean Salinity (SMOS) satellite. A priori model parameters were provided by in situ observations and gauging stations from the HYBAM Observatory. Our results show that the denitrification and the trace gas emissions present a strong cyclic pattern linked to the inundation processes that can be divided into three distinct phases: activation, stabilization and deactivation. We quantify the average yearly denitrification and associated emissions of CO2 and N2O over the entire watershed at 17.8 kgN ha−1 yr−1, 0.37 gC-CO2 m−2 yr−1 and 0.18 gN-N2O m−2 yr−1 respectively for the period 2011–2015. When compared to local observations, it was found that the CO2 emissions accounted for 0.01 % of the integrated ecosystem, which emphasizes the fact that minor changes to the land cover may induce strong impacts on the Amazonian carbon budget. Our results are consistent with the state of the art of global nitrogen models with a positive bias of 28 %. When compared to other wetlands in different pedoclimatic environments we found that the Amazonian wetlands have similar emissions of N2O with the Congo tropical wetlands and lower emissions than the temperate and tropical anthropogenic wetlands of the Garonne (France), the Rhine (Europe) and south-eastern Asia rice paddies. In summary our paper shows that a data-model-based approach can be successfully applied to quantify N2O and CO2 fluxes associated with denitrification over the Amazon basin. In the future, the use of higher-resolution remote sensing products from sensor fusion or new sensors like the Surface Water and Ocean Topography (SWOT) mission will permit the transposition of the approach to other large-scale watersheds in tropical environments.

Prioritizing coastal wetlands for marsh bird conservation in the U.S. Great Lakes

Human activity surrounding the Laurentian Great Lakes basin has significantly degraded coastal wetland habitats, resulting in severe marsh bird population declines and reduced coastal resilience to changing environmental conditions. Given the need to conserve remaining coastal wetlands for wildlife and people, we developed a spatial prioritization to identify the most important U.S. Great Lakes coastal wetlands for 14 marsh bird species. We modeled occurrence and relative abundance of each species using boosted regression trees, a machine learning algorithm, to relate standardized monitoring data to ten remotely-sensed environmental covariates. We then used Zonation conservation planning software to rank every wetland cell based on its importance for the suite of marsh bird species. Evaluation of the drivers of marsh bird occurrence and abundance revealed that open water, herbaceous wetland, latitude, longitude, and impervious surface were the most important predictors across focal species. The high-priority wetlands for marsh birds (defined as grid cells ranked in the top 20%) occurred along the shores of eastern Lake Ontario, western Lake Erie/St. Clair, Saginaw Bay, Green Bay, northern lakes Michigan and Huron, and western Lake Superior. Overall, less than half (42%) of high-priority coastal wetlands across the Great Lakes basin are currently under some level of protection, with Lake Ontario priority wetlands being the least protected (25%). Our findings represent an opportunity to improve coastal wetland conservation in a region where wetland loss and degradation continue to threaten marsh bird populations and the integrity of one of the world's largest freshwater ecosystems.

Past (1971–2018) and future (2021–2100) pan evaporation rates in the Czech Republic

Evaporation from open water surfaces is often estimated based on the pan evaporation (Epan), which is an essential measure for estimating atmospheric evaporative demand. Within the Central European region, Epan appears to be slightly underestimated in the case of the hydrological balance of water bodies. In the context of the recent multi-year period of drought, significant losses of surface water deposits were observed in countries of Central Europe. In spite of the ‘evaporation paradox’ phenomenon, Epan is not generally decreasing as expected by many studies from past decades. Recorded observations from the Czech Republic show an increase in Epan, which is associated with an increase in global radiation and vapor pressure deficit. The vast majority of meteorological stations show a strong or very strong increase in Epan during April, June, July and August. During the 1971–2018 period, the annual mean Epan has been increasing by an average of 2.97 mm yr−1. For the period 2001–2018, the mean Epan was 18% higher (519 mm) than the 1971–2000 average (440 mm). Our simulations of future scenarios, using regional climate models, predicted a growth in Epan of up to 27–54%. Such an increase in evaporation would cause serious consequences for surface water availability and agricultural production during the periods of drought in the Czech Republic, as the drought period 2014–2018 has clearly demonstrated.

DESENVOLVIMENTO HIDROLÓGICO E FENOLOGIA DE LAGOS DA PENÍNSULA FILDES – ANTÁRTICA

Este artigo avalia a dinâmica de congelamento e descongelamento dos lagos da península Fildes, utilizando imagens TerraSAR-X (TSX) entre fevereiro e abril de 2011, e a correlação da área de superfície líquida com as variáveis temperatura, precipitação e vento. A área de superfície líquida foi obtida a partir de classificação de imagem TSX, utilizando o classificador MAXVER, para 12 imagens do ano de 2011. A correlação de temperatura do ar atmosférico com a área, determinada pelo método de correlação de Spearman e Pearson, demonstra padrão de correlação significativa em 7 dos 15 lagos, enquanto a precipitação é uma importante variável na formação da superfície líquida em 11 dos 15 lagos. Todos os lagos voltados para a passagem de Drake mostram correlação significativa coma direção do vento que atua causando ondas na superfície líquida e que, por sua vez, provocam a quebra do gelo na superfície. Quanto à direção dos ventos, esta variável pode estar ligada à mudanças na duração do período de ablação para a península Fildes. De forma geral, as imagens TSX permitiram um detalhamento temporal inédito para a área de estudo, possibilitando mapear os lagos no período entre fevereiro e abril de 2011.

Potential evaporation dynamics over saturated bare soil and an open water surface

Actual evaporation (Ea) can be calculated as a fraction of potential evaporation (PE), which refers to the evaporation rate if supply water is unlimited. Potential evaporation depends on the available energy and the underlying material, and different approaches to estimate potential evaporation exist nowadays. This study provides a detailed analysis of the evaporation dynamics over fully saturated, sandy soil (PEs) and an open water surface (PEw). Moreover, the performance of commonly used methods to estimate PE is assessed. At the basis of these analyses is a lysimeter experiment in the Guanzhong Basin, China, which allowed a precise measurement of PE with a very high temporal resolution. Temperature profiles in lysimeters and meteorological data were also measured during the experiment. A comparison of PEs and PEw was carried out for seven consecutive days (August 11th to 17th, 2016). Results show that PEw is smaller than PEs on a daily scale, with PEw rates being bigger than PEs at night but smaller during daytime. Furthermore, the temporal dynamics of PEw lags 4–5 h behind PEs. In accordance with the energy balance equation, PE dynamics are mainly governed by “available energy”. The PE rates calculated by Penman-Monteith (PM) and Priestly-Taylor (PT) based on these measurements were also evaluated. The measured PE is relatively well reproduced by PM and PT equations. Finally, the effect of using different approaches to estimate PE on calculating Ea was tested by an integrated hydrological model that calculates water flow in the unsaturated zone by solving the Richards equation. The relative differences were up to 17.5%.

SIMULATION OF WINTER CIRCULATION OF THE ATMOSPHERE OVER THE BAIKAL REGION

In order to assess the influence of sources of pollutants on the atmosphere of the Lake Baikal water area, the ICMMG SB RAS model is adapted to the geographical and climatic conditions of the Baikal region. Two scenario calculations describing the characteristic circulation of the winter atmosphere in conditions when Lake Baikal is covered with ice and those in the presence of an open water surface were performed. Some variants of passive impurity transfer from Irkutsk by the North-West wind flow simulated in these scenarios are discussed. With the same large-scale input data, the impurity concentrations reaching the lake's water area were lower in the scenario with frozen Lake Baikal surface. A preliminary comparison of the results of calculations has shown that the influence of differences in the thermal properties of the surface of Lake Baikal in these scenarios is manifested in the atmosphere of the surrounding territory at distances of the order of the width of the lake.

Automatic Extraction of Open Water Using Imagery of Landsat Series

Open surface freshwater is an important resource for terrestrial ecosystems. However, climate change, seasonal precipitation cycling, and anthropogenic activities add high variability to its availability. Thus, timely and accurate mapping of open surface water is necessary. In this study, a methodology based on the concept of spatial autocorrelation was developed for automatic water extraction from Landsat series images using Taihu Lake in south-eastern China as an example. The results show that this method has great potential to extract continuous open surface water automatically, even when the water surface is covered by floating vegetation or algal blooms. The results also indicate that the second shortwave-infrared band (SWIR2) band performs best for water extraction when water is turbid or covered by surficial vegetation. Near-infrared band (NIR), first shortwave-infrared band (SWIR1), and SWIR2 have consistent extraction success when the water surface is not covered by vegetation. Low filter image processing greatly overestimated extracted water bodies, and cloud and image salt and pepper issues have a large impact on water extraction using the methods developed in this study.

Integration of hyperspectral and LiDAR data for mapping small water bodies

Inland water bodies are globally threatened by environmental degradation and climate change. On the other hand, new water bodies can be designed during landscape restoration (e.g. after coal mining). Effective management of new water resources requires continuous monitoring; in situ surveys are, however, extremely time-demanding. Remote sensing has been widely used for identifying water bodies. However, the use of optical imagery is constrained by accuracy problems related to the difficulty in distinguishing water features from other surfaces with low albedo, such as tree shadows. This is especially true when mapping water bodies of different sizes. To address these problems, we evaluated the potential of integrating hyperspectral data with LiDAR (hereinafter “integrative approach”). The study area consisted of several spoil heaps containing heterogeneous water bodies with a high variability of shape and size. We utilized object-based classification (Support Vector Machine) based on: (i) hyperspectral data; (ii) LiDAR variables; (iii) integration of both datasets. Besides, we classified hyperspectral data using pixel-based approaches (K-mean, spectral angle mapper). Individual approaches (hyperspectral data, LiDAR data and integrative approach) resulted in 2–22.4 % underestimation of the water surface area (i.e, omission error) and 0.4–1.5 % overestimation (i.e., commission error).The integrative approach yielded an improved discrimination of open water surface compared to other approaches (omission error of 2 % and commission error of 0.4 %). We also evaluated the success of detecting individual ponds; the integrative approach was the only one capable of detecting the water bodies with both omission and commission errors below 10 %. Finally, the assessment of misclassification reasons showed a successful elimination of shadows in the integrative approach. Our findings demonstrate that the integration of hyperspectral and LiDAR data can greatly improve the identification of small water bodies and can be applied in practice to support mapping of restoration process.

Prevalence of Intestinal Parasitic Infections and Transmission Risk Factors in Primary School Children in Mbeere North Sub-County, Embu County, Kenya

Intestinal parasitic infections (IPI) such as soil transmitted helminths (STH) and protozoa can cause diverse negative effects on human health. The prevalence of IPI in primary school children in Mbeere North sub-county in Embu County is not documented, though there are many cases reported in health centers in the area. The aim of this study was to establish the current prevalence of IPIs in primary school children in Mbeere North sub-county and the factors that perpetuate transmission which is necessary for implementation of suitable control programs in the study area. The study was done among primary school children from nine public schools. Three schools per administrative ward were randomly selected to represent each of the three administrative wards. A total of 414 pupils whose parents/guardians gave informed consent participated in the study, 46 pupils per school. Each participant provided a thumb size of early morning stool. The stool samples were processed by Formol-ether concentration technique and direct wet preparation methods for microscopic identification of intestinal parasites. A structured questionnaire was used to collect data on age, sex, sanitation and hygiene practices. The overall point prevalence of intestinal parasitic infections amongst the pupils was 43%. Chi-square (χ²) test was done to determine the relationship between prevalence and the pupils’ age and sex. There was no significant association between prevalence and sex (χ² = 0.184, P = 0.668, df = 1) but a significant association was found between prevalence and age where children 11 years and below were more pre-disposed than those above 11 years old (χ² =4.770, P=0.043, df =1). Similarly, a significant association was found between water source and prevalence of IPI; the parasites infection prevalence decreased when tap water was used and increased when open surface water was used (F = 6.15, P = 0.006). From this study, it is clear that IPIs particularly E. histolytica are a problem in primary school children in Mbeere North Sub-County. There is an urgent need for provision of safe domestic water in the county. Additionally, community education on sanitation and IPI transmission risk factors is urgently required. The information generated in this study is beneficial to the public health service in designing control strategies for areas of high transmission.

Surface water detection in the Caucasus

The Caucasus is an important global diversity hotspot and hosts a wide variety of surface water features, including major transboundary wetlands, in addition to large areas with irrigated agriculture and newly developed fishponds. In this study, we aim to establish the best performing methodology to produce surface water maps with a high degree of accuracy in the Caucasus. We evaluate optical data from Landsat 8 in both the dry and wet season for three study areas in the Caucasus. We test the performance of four different optical water indices derived from Landsat data, a method by Zou et al. (2017) also applied to Landsat data, and the European Commission Joint Research Centre (ECJRC) Global Surface Water dataset. We evaluate the performance of each water index using 5744 land cover validation/training points over all three study areas, which we manually classified by evaluating imagery from Google Earth. Using all validation points from all three study areas and both the wet and dry season, we find that the application of a logistic regression model using an optical surface water index (MNDWI) resulted in the most accurate open surface water maps. This approach achieved an overall accuracy of 93.0%, which is better than was found for freely available global surface water products.

Economic value of wetlands services in the Central Rift Valley of Ethiopia

This study estimates the economic values of wetlands services in the Central Rift Valley of Ethiopia. A choice experiment valuation method was used to quantify monetary value of wetlands. The study used data collected from 405 households complemented with data collected using Participatory Rural Appraisals. Multinomial logit and random parameter logit models were used to analyze the data. The results have demonstrated the economic values of wetlands in the study area. Households ascribe the highest value for biodiversity (marginal willingness to pay value of US $1.26 per percentage of biodiversity conserved), followed by water availability (US $0.87 per ha increase in open water surface area). The statistical inferences, however, showed significant heterogeneity among households in their preference for wetland management alternatives. The probability of choosing improved wetland management is significantly reduced by age of the household head and distance; while, it is increased by education, income and location. The finding also revealed an average and aggregate willingness to pay of US $7.5 and 694,141, respectively, for a change from status quo to high impact improvement scenario. It is, thus, recommended to take rehabilitation and conservation measures to ensure sustainable use and management of wetland resources in the area.

Evaluation of a new 18-year MODIS-derived surface water fraction dataset for constructing Mediterranean wetland open surface water dynamics

Wetlands are among the most biodiverse ecosystems in the world, due largely to their dynamic hydrology. Frequent observations by satellite sensors such as the Moderate Resolution Imaging Spectrometer (MODIS) allow for monitoring the seasonal, inter-annual and long-term dynamics of surface water extent. However, existing MODIS-based studies have only demonstrated this for large water bodies (>100 km2) despite the ecological importance of smaller-sized wetland systems. In this paper, we assessed if the temporal dynamics of surface water extent can be effectively captured with MODIS for a wide range of water body sizes, specifically 0.01 km2 and larger. We constructed the temporal dynamics of surface water extent for 340 individual water bodies in the Mediterranean region between 2000 and 2017, using a previously developed 8-day 500 m MODIS surface water fraction (SWF) dataset. We then evaluated how MODIS SWF time series corresponded to water extent derived from a Landsat-based dataset and to satellite-altimetry derived water level data. Results showed that although correlations between MODIS- and Landsat-derived SWF were poor for relative static water bodies (r = 0.17), they increased for more dynamic water bodies (r = 0.81). For dynamic water bodies>100 km2, time series of water extent derived from MODIS SWF showed good correlation with both Landsat (r ≥ 0.76) and water level data (ρ ≥ 0.63). Our MODIS SWF dataset can also effectively monitor the variability of very small water bodies (<1 km2) when comparing with Landsat data as long as the temporal variability in their surface water area was high. We conclude that MODIS SWF is a useful product to help understand hydrological dynamics for both small and larger-sized water bodies, and to monitor their seasonal, intermittent, inter-annual and long-term changes.

Mapping and sampling to characterize global inland water dynamics from 1999 to 2018 with full Landsat time-series

Global surface water extent is changing due to natural processes as well as anthropogenic drivers such as reservoir construction and conversion of wetlands to agriculture. However, the extent and change of global inland surface water are not well quantified. To address this, we classified land and water in all 3.4 million Landsat 5, 7, and 8 scenes from 1999 to 2018 and performed a time-series analysis to produce maps that characterize inter-annual and intra-annual open surface water dynamics. We also used a probability sample and reference time-series classification of land and water for 1999–2018 to provide unbiased estimators of area of stable and dynamic surface water extent and to assess the accuracy of the surface water maps. From the reference sample data, we estimate that permanent surface water covers 2.93 (standard error ±0.09) million km2, and during this time period an estimated 138,011 (±28,163) km2 underwent only gain of surface water, over double the estimated 53,154 (±10,883) km2 that underwent only loss of surface water. The estimated area of 950,719 (±104,034) km2 that experienced recurring change between land and water states is far greater than the area undergoing these unidirectional trends. From a probability sample of high resolution imagery, an estimated 10.9% (±1.9%) of global inland surface water is within mixed pixels at Landsat resolution indicating that monitoring of surface water changes requires improved spatial detail. We provide the first unbiased area estimators of open surface water extent and its changes with associated uncertainties and illustrate the challenges of tracking changes in surface water area using medium spatial and temporal resolution data.

Mineralogical and geochemical characterisation of alkaline lake sediments to trace origin, depositional processes, and anthropogenic impacts: Inle Lake (Southern Shan State, Myanmar)

Southeast Asia contains a major portion of wetland areas of international significance, but only 14% of these wetlands are under protection. Inle Lake is the second largest freshwater body in Myanmar and the most important for its environmental, economic, and agricultural values. However, the lake is seriously threatened by anthropic activities as they are dramatically decreasing its open water surface. While water-quality studies have assessed the presence of nutrients and contaminants, little is known about the sediment quality. Therefore, the overall objective of the present study was to analyse the natural sedimentation patterns and the anthropogenic disturbances in this lake. To this end, both grab and core sediments were investigated by combining mineralogical, morphological, and geochemical analyses, and their composition was compared to the composition of rocks and soil from the watershed. Mineralogical data indicate that sediment samples collected at or near the main inflow, and at the outflow show a lower abundance (about 43%) of calcite, which generally increases with depth in sediment cores, reaching more than 90% in the centre of the lake. Lower calcite contents correspond to higher percentages of other minerals. These findings allow us to clearly distinguish the allogenic (quartz, mica/illite, kaolinite, and detrital calcite) from the endogenic (mostly calcite) minerals, as confirmed by the soil and rock mineralogy. The origin of the mineral contents at the different sites is further supported by the observation of the grain morphology and composition. The most abundant metal in the lake sediments is Ca, in agreement with the high calcite content. Following in order of abundance are Al, Fe, and K, which are connected to the detrital input to the lake. In the rock and soil samples, the most abundant trace element is Mn, which is significantly correlated with many other trace elements (e.g., Co, Cr, Cu, Ni, V, Zn, and As), suggesting their common lithogenic origin. Similarly, in the lake sediments, the trace elements Co, Cr, Cu, Ni, Pb, V, Zn, and As display fairly homogenous concentration ranges, in the order of tens of ppm, and are strongly correlated with the detrital minerals (mica, kaolinite, and quartz) and negatively correlated with the calcite content. The mineralogical and elemental distribution in the lakebed area and with depth in the sediment cores allowed us to identify the increased soil and bedrock erosion as a major anthropogenic impact to the lake. The sedimentation disturbance is especially evident at the main inflow, the Nanlit stream, which has caused the development of a large alluvial fan, visible in satellite images. This large sediment input is likely to have accelerated the decrease in open water surface and in depth of this shallow lake. Additionally, when the measured concentrations of Potentially Toxic Elements were compared to consensus-based Sediment Quality Guidelines (SQGs), it was shown that As, Cr, Cu, Ni, Pb, and Zn often exceeded the threshold effect concentration (TEC), and that Cr, Ni, and Pb sometimes exceeded the probable effect concentration (PEC). On the other hand, Enrichment Factors (EFs), calculated with respect to soils of the watershed, showed minimal enrichment and indicated that the potential toxicity effect was primarily a result of detrital sedimentation outpacing carbonate sedimentation. Sediment characterisation allowed us to obtain a better understanding of lake dynamics and environmental impacts, which is fundamental in achieving effective management and maintenance of the ecological character of Inle Lake, in line with the objective set by the Ramsar Convention for wetlands of international relevance.

Open water detection in urban environments using high spatial resolution remote sensing imagery

Commonly applied water indices such as the normalized difference water index (NDWI) and the modified normalized difference water index (MNDWI) were originally conceived for medium spatial resolution remote sensing images. In recent decades, high spatial resolution imagery has shown considerable potential for deriving accurate land cover maps of urban environments. Applying traditional water indices directly on this type of data, however, leads to severe misclassifications as there are many materials in urban areas that are confused with water. Furthermore, threshold parameters must generally be fine-tuned to obtain optimal results. In this paper, we propose a new open surface water detection method for urbanized areas. We suggest using inequality constraints as well as physical magnitude constraints to identify water from urban scenes. Our experimental results on spectral libraries and real high spatial resolution remote sensing images demonstrate that by using a set of suggested fixed threshold values, the proposed method outperforms or obtains comparable results with algorithms based on traditional water indices that need to be fine-tuned to obtain optimal results. When applied to the ASTER and ECOSTRESS spectral libraries, our method identified 3677 out of 3695 non-water spectra. By contrast, NDWI and MNDWI only identified 2934 and 2918 spectra. Results on three real hyperspectral images demonstrated that the proposed method successfully identified normal water bodies, meso-eutrophic water bodies, and most of the muddy water bodies in the scenes with F-measure values of 0.91, 0.94 and 0.82 for the three scenes. For surface glint and hyper-eutrophic water, our method was not as effective as could be expected. We observed that the commonly used threshold value of 0 for NDWI and MNDWI results in greater levels of confusion, with F-measures of 0.83, 0.64 and 0.64 (NDWI) and 0.77, 0.63 and 0.59 (MNDWI). The proposed method also achieves higher precision than the untuned NDWI and MNDWI with the same recall values. Next to numerical performance, the proposed method is also physically justified, easy-to implement, and computationally efficient, which suggests that it has potential to be applied in large scale water detection problem.

Floating Azolla Cover Influences Evapotranspiration from Flooded Water Surfaces

Floating vegetation is a common sight in flooded environments and plays a key role in regulating the global water balance, particularly through evapotranspiration (ET). Azolla is a dominant floating fern in lowland paddy fields, and its biological characteristics contribute mainly to nitrogen (N) fertilization. Here, to determine the potential of the rapidly growing Azolla as an alternative N source and its role in water conservation in flooded paddy ecosystems, we investigated its productivity and the resulting reduction in ET in two separate pot experiments. Azolla cover significantly decreased ET losses under high and low flooding depths compared with open water surfaces and green polyester covered mats (analogous to plant cover). Additionally, Azolla showed significantly higher biomass productivity, carbon assimilation, and N accumulation in presence of phosphorous (P) than in its absence under both high and low flooding depths. Overall, our results indicate that the efficiency of Azolla as an N alternative in agricultural ecosystems can be realized in presence of P, regardless of the flooding water depth. Additionally, we predict a relative reduction in the ET efficiency of Azolla in rice paddies. Field studies are, however, necessary to confirm these findings.

Spatial and Temporal Dynamics of Urban Wetlands in an Indian Megacity over the Past 50 Years

Asian megacities have attracted much scientific attention in the context of global urbanization, but few quantitative studies analyze wetland transformation in the rural–urban interface. With its rampant growth and transformation from a tree-lined “Garden City” to a busy megalopolis with often-blocked highways and large built-up areas, Bengaluru (Karnataka, S-India) is a good example for assessing how urbanization has led to the acute degradation of wetlands. We therefore investigated long-term land cover and wetland changes from 1965 to 2018 based on an object-based classification of multi-temporal Corona and Landsat images. To quantify and compare the dynamics of open water surfaces and vegetation, we defined the potential wetland areas (PWA) along the rural–urban gradient and linked our analyses to an index describing the degree of urbanization (survey stratification index (SSI)). During the five decades studied, built-up areas in the Bengaluru Urban district increased ten-fold, with the highest growth rate from 2014 to 2018 (+ 8% annual change). Patches of lake wetlands were highly dynamic in space and time, partly reflecting highly variable annual rainfall patterns ranging from 501 mm in 1965 to 1374 mm in 2005 and monsoon-driven alterations in the hydrologic regime. While water bodies and flooded areas shrunk from 64 km2 in 1965 to 55 km2 in 2018, in 1965, the total rural wetland area with an SSI &gt; 0.5 was twice as high as in 2018. The rural–urban land cover pattern within potential wetland areas changed drastically during this period. This is reflected, for example, by a four-fold increase in the wetland area with an SSI of 0.3, as compared to a decline by 43% in wetland area with an SSI of 0.8. While, in urban areas, wetlands were mostly lost to construction, in areas with a rural character, open water bodies were mainly transformed into green space. The detected changes in urban wetlands were likely accompanied by ecological regime changes, triggering deteriorations in ecosystem services (ESS) which merit further research.

Open Water Flow in a Wet/Dry Multiply-Connected Channel Network: A Robust Numerical Modeling Algorithm

Our goal was to develop a robust algorithm for numerical simulation of one-dimensional shallow water flow in a complex multiply-connected channel network with arbitrary geometry and variable topography. We apply a central-upwind scheme with a novel reconstruction of the open water surface in partially flooded cells that does not require additional correction. The proposed reconstruction and an exact integration of source terms for the momentum conservation equation provide positivity preserving and well-balanced features of the scheme for various wet/dry states. We use two models based on the continuity equation and mass and momentum conservation equations integrated for a control volume around the channel junction to its treatment. These junction models permit to simulate subcritical and supercritical flows in a channel network. Numerous numerical experiments demonstrate the robustness of the proposed numerical algorithm and a good agreement of numerical results with exact solutions, experimental data, and results of the previous numerical studies. The proposed new specialized test on inundation and drying of an initially dry channel network shows the merits of the new numerical algorithm to simulate the subcritical/supercritical open water flows in the networks.