Runoff mechanisms

The processes through which rainfall is turned into runoff, together with the nature of the material through which water moves, control the chemical characteristics of streamflow. Specific runoff mechanisms operating in a landscape control the flowpaths by which water moves through the landscape. Flowpath-depen-dent differences, such as the total time that water spends in contact with different soil horizons or bedrock (residence time), can strongly influence runoff amounts and timing, the relative contribution of event (new) versus stored (old) water, and runoff chemistry. 

Transport in water is an important mechanism for transfer of biogeochemical elements between the atmosphere, land, and oceans. In particular, rain is the primary means of removal from the atmosphere for many substances, and rivers (and to some extent groundwater) convey weathering products and runoff from the land surface to the oceans. 

The combined influences of runoff generation mechanisms, runoff flowpaths, and soil properties together control runoff chemistry. In spite of the wide range of interactions that characterize terrestrial environments, a few broad generalities can be offered, as the chemical composition of streamflow typically contains... 

Dinitrophenol is a member of the aromatic family of pesticides, many of which exhibit insecticide and fungicide activity. DNP is considered to be highly toxic to humans, with a lethal oral dose of 14 to 43mg/kg. Environmental exposure to DNP occurs primarily from pesticide runoff to water. DNP is used as a pesticide, wood preservative, and in the manufacture of dyes. DNP is an uncoupler, or has the ability to separate the flow of electrons and the pumping of ions for ATP synthesis. This means that the energy from electron transfer cannot be used for ATP synthesis [75,77]. The mechanism of action of DNP is believed to inhibit the formation of ATP by uncoupling oxidative phosphorylation. 

GEOTRANSF is a semi-distributed modelling system which is built around a conceptual model that limits the model s complexity while maintaining an accurate description of the main mechanisms controlling runoff production and transfer at the basin scale [43]. With this structure GEOTRANSF can be integrated with... 

In watersheds where surface runoff is limited by infiltration rate rather than soil-water storage capacity, areas of the watershed can alternate between sources and sinks of surface flow. This again will be a function of soil properties, rainfall intensity and duration, and antecedent moisture condition. As surface runoff is the main mechanism by which phosphorus is exported from most watersheds, it is clear that, if surface runoff does not occur, phosphorus export can be small. Thus, consideration of hydro-logic pathways and variable source areas is critical to a more detailed understanding of phosphorus export from agricultural watersheds. 

There are several drainage mechanisms are employed at petroleum and related facilities - surface runoff or grading, spill containment (diking), gravity sewers (oily water and sanitary) and pressurized sewer mains, and lift station collection sumps. 

Regulation of expression may occur at both the transcriptional and post-transcriptional levels. The mRNA for GM-CSF contains (in common with those of some other cytokines) conserved regulatory sequences in the 3 untranslated region, which may affect its rate of translation. The gene is constitutively transcribed in monocytes, endothelial cells and fibroblasts, but the mRNA is unstable and so does not accumulate to levels sufficient to allow translation into significant amounts of protein. Activation of these cells results in the increased expression of GM-CSF protein, which arises from both an enhanced rate of transcription (as detected in nuclear runoff experiments) and also an increased stability of the mRNA, perhaps by mechanisms analogous to those described above during activation of G-CSF expression ( 2.2.3.1). 

Trace elements are discharged into the ocean in particulate and dissolved form as a component of river runoff and groundwater seeps. They are introduced into these waters during the chemical and mechanical weathering of crustal rocks. Thus, the chemical composition of river water is dependent on the composition of the rocks in the... 

The formation of substantial evaporite deposits requires two conditions (1) some mechanism by which salt ion concentrations are kept at supersaturated levels, and (2) a steady resupply of salt ions. The hydrogeologic setting that is most likely to meet these two criteria are shallow-water embayments located in arid climates where sea level is relatively stable and terrestrial runoff is very low or absent. 

Shallow-water embayments provide a mechanism to isolate seawater so that evaporation can raise salt ion concentrations. Arid climates are required to ensure that the rate of water loss from evaporation exceeds the rate of water supply by rainfell, groundwater seeps, or river runoff. Seawater can be resupplied continuously via a type of antiestuar-ine circulation as illustrated in Figure 17.2 or episodically as a result of sea level change, plate tectonics, or very high tides and storm surges. 

A landfill can be seen as a spatially heterogeneous porous and, by design, unsaturated system. Flow paths and physical mechanisms of runoff generation play a crucial role for the hydrology and ultimately leaching reactions within a landfill. Figure 1 illustrates the flow... 

Shorter fallow periods store precipitation more efficiently for several reasons. First, water is stored more readily when soils are dry, early in fallow periods, than later in fallow periods when soils are wetter. Secondly, when plant residues are preserved with no-till methods, water infiltration is encouraged and runoff and evaporative losses are reduced as compared to tilled soils with less residue. Indeed, mechanical tillage within fallow periods practically assures soil water loss as soils dry out and warm up in the tilled zone. 

The mechanism of the transfer of the eustatic component of the world ocean level rise into the Black Sea and the Sea of Azov is an intricate and poorly understood process. It is clear that in these seas the eustatic rise differs from that in the ocean as a whole. The main cause is relative isolation of these seas from the ocean. Thus, the volumetric changes in the sea water levels are likely to be more susceptible to changes in the ratio between the components of the water balance of the sea (river runoff, precipitation, and evaporation) than to the eustatic rise in the ocean level. 

When rain falls over land some drain off the surface directly into surface water courses in surface runoff. A further part of the incoming rainwater percolates into the soil and passes more slowly into either surface waters or underground reservoirs. Water held in rock below the surface is termed groundwater, and a rock formation that stores and transmits water in useful quantities is termed an aquifer. Water that passes through soil or rock on its way to a river is chemically modified during transit, generally by addition of soluble and colloidal substances washed out of the ground. Some substances are removed from the water for example, river water often contains less lead than rainwater one mechanism of removal is uptake by soil. 

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