Simulating hydrological and nonpoint source pollution processes in a karst watershed: A variable source area hydrology model evaluation

AbstractAn ecohydrological watershed model can be used to develop an efficient watershed management plan for improving water quality. However, karst geology poses unique challenges in accurately simulating management impacts to both surface and groundwater hydrology. Two versions of the Soil and Water Assessment Tool (SWAT), Regular-SWAT and Topo-SWAT (which incorporates variable source area hydrology), were assessed for their robustness in simulating hydrology of the karstic Spring Creek watershed of Centre County, Pennsylvania, USA. Appropriate representations of surface water – groundwater interactions and of spring recharge – discharge areas were critical for simulating this karst watershed. Both Regular-SWAT and Topo-SWAT described the watershed discharge adequately with daily Nash-Sutcliffe efficiencies (NSE) ranging from 0.77 to 0.79 for calibration and 0.68–0.73 for validation, respectively. Because Topo-SWAT more accurately represented measured daily streamflow, with statistically significant improvement of NSE over Regular-SWAT during validation (p-value=0.05) and, unlike Regular-SWAT, had the capability of spatially mapping recharge/infiltration and runoff generation areas within the watershed, Topo-SWAT was selected to predict nutrient and sediment loads. Total watershed load estimates (518t nitrogen/year, 45t phosphorus/year, and 13600t sediment/year) were within 10% of observed values (−9.2% percent bias for nitrogen, 6.6% for phosphorous, and 5.4% for sediment). Nutrient distributions among transport pathways, such as leaching and overland flow, corresponded with observed values. This study demonstrates that Topo-SWAT can be a valuable tool in future studies of agricultural land management change in karst regions

Reviews and Syntheses: Ocean acidification and its potential impacts on marine ecosystems

Ocean acidification, a complex phenomenon that lowers seawater pH, is the net outcome of several contributions. They include the dissolution of increasing atmospheric CO2 that adds up with dissolved inorganic carbon (dissolved CO2, H2CO3, HCO3−, and CO32−) generated upon mineralization of primary producers (PP) and dissolved organic matter (DOM). The aquatic processes leading to inorganic carbon are substantially affected by increased DOM and nutrients via terrestrial runoff, acidic rainfall, increased PP and algal blooms, nitrification, denitrification, sulfate reduction, global warming (GW), and by atmospheric CO2 itself through enhanced photosynthesis. They are consecutively associated with enhanced ocean acidification, hypoxia in acidified deeper seawater, pathogens, algal toxins, oxidative stress by reactive oxygen species, and thermal stress caused by longer stratification periods as an effect of GW. We discuss the mechanistic insights into the aforementioned processes and pH changes, with particular focus on processes taking place with different timescales (including the diurnal one) in surface and subsurface seawater. This review also discusses these collective influences to assess their potential detrimental effects to marine organisms, and of ecosystem processes and services. Our review of the effects operating in synergy with ocean acidification will provide a broad insight into the potential impact of acidification itself on biological processes. The foreseen danger to marine organisms by acidification is in fact expected to be amplified by several concurrent and interacting phenomena

Mechanistic insight into salt tolerance of Acacia auriculiformis: The importance of ion selectivity, osmoprotection, tissue tolerance, and Na+ exclusion

10.3389/fpls.2017.00155Frontiers in Plant Science815

Plant–water relations in subtropical maize fields under mulching and organic fertilization

The plant–water relationship of maize under conservation practices needs to be assessed to quantify the effectiveness of the practices in conserving soil water for crop production. This study evaluated in three trials how straw and plastic film mulching and organic manure application could potentially change water fluxes in the root zone and increase maize yield. A mathematical model HYDRUS-1D was calibrated against the observed soil water content and drainage data to predict the water fluxes in the root zone soil. The model simulated soil water dynamics in the root zone with satisfactory performance (RMSE of 0.6–2.3%, CD of 0.37–1.41, NSE of 0.18–0.88, and R2 of 0.62–0.91) during both the calibration and validation periods. The model predicted the observed drainage in a lysimeter with only a 5.5–11.7% bias and actual evapotranspiration (ETc) with a 2.6–6.7% bias for the control conditions in all three trials when the model was provided with measured plant growth, soil properties, and weather data. Both measurement and modeling confirmed that mulching augmented soil water storage by reducing ETc, i.e., 0.24–0.37 mm d-1 by straw mulching and 0.05–0.24 mm d-1 by plastic mulching during the trials. Manure application did not affect the ETc rate and resulted in the highest grain yield (6.8–8.3 Mg ha˗1) followed by plastic mulching (6.1–8.1 Mg ha˗1) and straw mulching (5.3–7.5 Mg ha˗1). Manure application increased the harvest index by optimally allocating biomass because of a steady supply of water and nutrients. The straw mulch, plastic mulch, and manure treatments increased grain yield by 13%, 24%, and 35%, respectively, compared to the control condition. Large-scale implementation of these practices would lessen blue water scarcity in agriculture

Phototransformation Induced by HO Radicals

The hydroxyl radical (HO•) is a strong oxidizing agent that can transform dissolved organic matter (DOM) into many intermediate photoproducts and byproducts, which include low molecular weight (LMW) DOM, hydrogen peroxide (H2O2), CO2, dissolved inorganic carbon (DIC: dissolved CO2, H2CO3, HCO3−, and CO32−), NO3−, NH4+, and so on, in surface waters. Furthermore, HO• has a prominent role in the transformation of recalcitrant pollutants into more biodegradable compounds. This chapter discusses the key sources of HO• in natural waters and elucidates the phototransformation pathways of high molecular weight (HMW) and low molecular weight (LMW) DOM induced by HO•. As an example, the photooxidation of methylmercury chloride (MeHgCl) by hydroxyl radicals is discussed along with its reaction mechanism. Among the possible indicators of DOM transformation, the decrease in the fluorescence intensity of autochthonous fulvic acid is discussed based on field observations. The presented results suggest that HO• may be involved in the photooxidation of both HMW and LMW DOM in surface waters

Spatial and temporal variability of 0‐ to 5‐m soil–water storageat the watershed scale

Dynamic relationships among rainfall patterns, soil water distribution, and plant growth are crucial for sustainable conservation of soil and water resources in water-limited ecosystems. Spatial and temporal variation in deep soil water content at a watershed scale have not yet been characterized adequately due to the lack of deep soil water data. Deep soil-water storage (SWS) up to a depth of 5m (n=73) was measured at 19 sampling occasions at the LaoYeManQu watershed on the Chinese Loess Plateau (CLP). At a depth of 0-1.5m, the annual mean SWS was highly correlated with rain intensity, and the correlation decreased with depth, but within the layers at 1.5-5.0m, the changes in SWS indicated a lag between precipitation and the replenishment of soil water. Geostatistical parameters of SWS were also highly dependent on depth, and the mean SWS presented similar spatial structures in two adjacent layers. Temporal stability of SWS as indicated by mean relative difference, standard deviation of the relative difference (SDRD), and mean absolute bias error (MABE) was significantly weaker at the shallow than at deeper layers. Soil separates and organic carbon content controlled the spatial pattern of SWS at the watershed scale. One representative location (Site 57) was identified to estimate the mean SWS in the 1- to 5-m layer of the watershed. Semivariograms of the SDRD and MABE were best fitted by an isotropic spherical model, and their spatial distributions were depth-dependent. Both temporal stability and spatial variability of SWS increased over depth. This study is helpful for deep SWS estimation and sustainable management of soil and water on the CLP, and for other similar regions around the world

Insights into solubility of soil humic substances and their fluorescence characterisation in three characteristic soils

Soil humic substances (HS) are involved in almost all biogeochemical processes and functions in soils, thus their extraction from aiming to their characterization is very important. However, many factors that influence HS extraction from soil still need further studies. The aim of this work was to assess and quantify comparatively the solubility of soil HS as a function of extraction time, various extractants, solid to liquid ratio and sequential extraction. In this work three different soils, i.e. a forest, a maize and a paddy soil, were examined to assess the solubility of soil HS based on their fluorescence (excitation-emission matrix, EEM) features and changes in nutrient (NO3−-N, PO43−-P and dissolved Si) contents using multiple extraction approaches (time-dependent, various extractants, solid to liquid ratio, and sequential extraction). Three fluorescent components, i.e. humic acids-like (HA-like), fulvic acids-like (FA-like), and protein-like fluorophores (PLF), were identified by parallel factor (PARAFAC) analysis of EEM spectra of the various soil extracts. The solubility of HS, dissolved organic carbon (DOC) and nutrients were shown to increase with extraction time, except for PLF. The FA-like fraction disappeared completely in KCl extracts of all three soils, suggesting the inefficiency of salt extraction. Conversely, HS and nutrients solubility substantially increased in alkaline extracts, and dissolved Si was correlated significantly with the fluorescent intensities of HA-like and FA-like, thus confirming the well-known typical process of alkaline dissolution of HS bound to phytolith and silicate minerals. The relative solubility of HS and nutrients was higher at lower solid to liquid ratio (1:250–1:100), whereas their maximum yields was achieved at high solid to liquid ratio (1:10) for all three soils. Sequential extraction results showed that the first water extraction step contributed 42–55% of HS, which suggested that a single extraction was insufficient to recover HS. In conclusion, water and alkaline extraction could provide, respectively, the labile and insoluble complexed HS existing in soil