Hydrological functions

Further support of rapid recovery of hydrological functions in secondary forest comes from the Zona Bragantina, east of Belem. Here, Holscher et al. (1997) used a Bowen ratio approach to measure evapotranspiration in a 2.5-3.5-year-old secondary forest on abandoned crop land, and found an average daily rate (3 9 mm d ) very close to that of mature forests receiving similar amounts of radiation. During the dry season, this secondary forest was absorbing water from below 3 m depth, and therefore had recovered a portion of its deep soil water uptake capacity. 

Kuznetsov NT (1991) Geographical-ecological aspects of hydrological functions of the Aral Sea. Izv. AN SSSR, Ser Geogr 4 82-88 (in Russian)... 

Abstract In most of the world s watercourses, dramatic modifications have occurred as a consequence of intensive use by human societies. The simplification of the channel network and the alteration of water fluxes have an impact upon the capacity of fluvial systems to recover from disturbances, because of their irreversible consequences. However, human impacts on river hydrology, such as those that derive from regulating their flow or by affecting their channel geomorphology, affect the functional organisation of streams, as well as the ecosystem services that derive from them, and lead to the simplification and impoverishment of these ecosystems. 

Alterations of water flow, independent of the cause, impact the stmcture and function of aquatic ecosystems. Extended drought produces the loss of hydrologic connectivity between stream compartments, and affects the biota. Therefore, flow cessation triggers a chain of cascading effects, eventually affecting community structure and ecosystem functioning. 

Terrestrial ecosystems (plants and animals) under water scarcity suffer from water stress, and aquatic ecosystems of intermittency in water flow. Water scarcity has implications on hydrologic resources and systems coimectivity, as well as negative side-effects on biodiversity, water quality, and river ecosystem functioning. Finally, water scarcity has also direct impacts on citizens and economic sectors that use and depend on water, such as agriculture, tourism, industry, energy and transport. 

The ET cover cannot be tested at every landfill site so it is necessary to extrapolate the results from sites of known performance to specific landfill sites. The factors that affect the hydrologic design of ET covers encompass several scientific disciplines and there are numerous interactions between factors. As a consequence, a comprehensive computer model is needed to evaluate the ET cover for a site.48 The model should effectively incorporate soil, plant, and climate variables, and include their interactions and the resultant effect on hydrology and water balance. An important function of the model is to simulate the variability of performance in response to climate variability and to evaluate cover response to extreme events. Because the expected life of the cover is decades, possibly centuries, the model should be capable of estimating long-term performance. In addition to a complete water balance, the model should be capable of estimating long-term plant biomass production, need for fertilizer, wind and water erosion, and possible loss of primary plant nutrients from the ecosystem. 

Fig. 8.42 Kerosene residual content (KRC) of soils as a function of clay and moisture contents. Reprinted from Fine P, Yaron B (1993) Outdoor experiments on enhanced volatilization by venting of kerosene components from soil. J Contam Hydrology 12 335-374. Copyright 1994 with permission of Elsevier...

Evaporation and Evaporative Fractionation of Water. Evaporation from standing water bodies is the principal fractionation mechanism in most hydrological systems. Evaporative isotopic enrichment is a function of numerous factors (e.g., temperature, salinity, and relative humidity) that cause considerable variation in the lsO/ ieO and D/H ratios of natural surface waters. Craig and Gordon (22) evaluated isotopic effects on precipitation and evaporation in the ocean-atmosphere system. Much of what was developed in that work is directly applicable to the freshwater systems discussed here. 

The variability of the P content of fulvic acids appears to be a function of both the hydrologic conditions and the nutrient status of the aquatic system in which the fulvic acids were formed. McKnight et al. (1985) hypothesized that the P in Thoreau s Bog fulvic acids was in the form of organic phosphate esters or inositol phosphates, which are major phosphorus products from the breakdown of plant material. The solubility of these P-containing moieties would be greater in bogwater than in most surface... 

---