The articles in this issue are a selection of the presentations made at the 2nd International Conference of Biohydrology. This special issue ‘Biohydrology—coupling biology and soil hydrology from pores to landscapes’ contains a range of articles on biological and hydrological interactions in soil, including large-scale systems research on the influence of forests on catchments and small-scale reductionist research on processes operating at the scale of soil pores. Copyright © 2010 John Wiley & Sons, Ltd.
WHAT IS BIOHYDROLOGY?
The etymology of ‘Biohydrology’ is simple (Bios = life, hydōr = water, logos = study) and subtly different from ‘Ecohydrology’, but in practice there is a considerable overlap between these disciplines. To some extent, a biohydrologist may take a reductionist approach, whereas the ecohydrologist will favour a systems approach, similar to the difference between a biologist and ecologist. This is reflected in the definitions that exist for both terms:
Biohydrology is the study of the interactions between the water cycle and plants and animals (source: science-dictionary.com).
Ecohydrology is the study of the interactions between water and ecosystems (source: Wikipedia).
However, in both disciplines there is a common interdisciplinary approach with an ultimate goal of understanding the interactions between living organisms and hydrology. Whether a hydrologist or soil physicist with an interest in biology, ecology, forestry, agriculture, and so on is ‘bio’ or ‘eco’ is splitting hairs.
THE NATURE AND SCOPE OF THIS SPECIAL ISSUE
During the European Geosciences Union (EGU) 2005 General Assembly in Vienna, the convenors and participants of related sessions (Dekker, Doerr, Hallett, Lichner, Ritsema and Sir) proposed an international conference that would draw together scientists interested in biological and hydrological interactions in soil. This culminated in ‘Biohydrology 2006’ that attracted over 90 participants from 16 countries to Prague. There was great diversity in the presentations, ranging in scale from microbial interactions with the air–water interface in soil pores to the impact of forests on catchment hydrology. Interest in the meeting grew and by the time Biohydrology 2009 was held in Bratislava, there were over 130 participants from 33 countries. The diversity of topics also grew, as did the participation of non-governmental organizations and policy bodies interested in protecting water resources. Information about the conferences can be found at http://www.ih.savba.sk/biohydrology2006/ and http://www.ih.savba.sk/biohydrology2009/, respectively.
This special issue contains a small selection of the presentations from Biohydrology 2009. It provides a representative cross-section of topics that were discussed at the conference, ranging from reductionist research on the interaction between specific organisms and hydrological processes to systems research that encompasses a vast array of biological and hydrological processes operating at much larger scales.
There is now irrefutable evidence of man's slow destruction of nature and the impacts on hydrological processes. Lacombe et al. (2010) provide a fascinating hydrological study of one of the most extreme examples from recent memory, the Vietnam War. Over three times the bomb tonnage was dropped compared to World War II, with added impacts on vegetation from the use of defoliants and population exodus. Forests were targeted so that the enemy could be exposed and the consequence of war was the abandonment of many agricultural fields. Hydrometerological data are presented for two catchments of the Mekong basin in Laos that were either heavily bombed or depopulated. The impacts were rapid, dramatic and slow to recover. One lesson that Lacombe et al. (2010) hopes is learnt from this study is the potential impact of current large-scale forestry operations in southeast Asia.
Armies of ants can also impact hydrology, as demonstrated by Cerdà and Doerr (2010) following wildfire on scrubland in the Mediterranean climate of southern Spain. Ant mounds were found to be highly erodible and caused an initial increase in overland flow because they were less permeable. Over time, however, macropores on control plots without ants became clogged with ash material, whereas mounds retained conductive pathways that maintained infiltration rates. On rangelands in the same region, Ruiz-Sinoga et al. (2010) demonstrated that grazing increases run-off. They studied several sites across a climatic gradient to obtain data with different rainfall patterns and topsoil moistures. Interestingly, the driest region studied had large amounts of run-off when the soil was dry because of the development of water repellency. The consequences were discussed in terms of the potential implications of drier conditions due to climate change. Lichner et al. (2010) also observed a large interaction between soil moisture, vegetation and water repellency for grass, glade and forest sites in the continental climate of Slovakia. After long periods of hot and dry weather, water repellency increased under all vegetation covers, with the impact evident in water transport measurements. They discuss the potential implications of these findings in the context of climate change predictions for this region.
Field research by Cammeraat et al. (2010) in southern Spain focused on the impact of abandoning ploughed agricultural land on water infiltration patterns, ponding and run-off. Stem-root flow developed within 12 years of land abandonment, resulting in the deep channelling of water into soil beneath vegetation tussocks. This process was also observed on natural land but not in a field that was still being ploughed. Nadezhdina et al. (2010) also investigated the impact of vegetation on hydrology, this time focusing on hydraulic redistribution by trees. From sap-flow measurements in roots and stems of several forest species and sites across Europe they demonstrated water movement by roots from drier to wetter regions of soil and a downward movement of water from stems under not only wet conditions from the channelling of atmospheric moisture but also in drought or frosty conditions from water stored in above-ground tree tissues. Irrigation water was also redistributed laterally in soil by roots.
Changes in vegetation and hydrology may also influence atmospheric conditions, as shown by Kovářová and Pokorný (2010) for a drying wetland with decreasing vegetation cover. They examined meteorological data from several Czech weather stations located in different environments. Temperature increases were 1·8 °C from 1961–1970 to 2001–2006 for the disturbed wetland region, which was about 1 °C greater than for a forest region and an urban region. There were also 15 fewer precipitation days between these sampling phases in the wetland region, whereas they remained almost unchanged elsewhere. However, the importance of ecology to hydrological processes was refuted by Bayabil et al. (2010) who found that in the Ethiopian highlands, topography had such a great impact that ecology was insignificant. They observed infiltration rates greater than rainfall intensity, so overland flow would not result. When infiltration is limited, ecological processes to improve infiltration rates were thought to be an important mitigation strategy to avoid erosion.
Transpiration from trees was examined by Sellin and Lubenets (2010), who took a range of tree and canopy measurements in a silver birch forest over the April to October growing season in Estonia. A very important finding was that night-time transpiration occurred in this humid temperate environment (a process generally thought to only occur in arid environments). They also demonstrated that the influence of the physiological properties of trees, such as stomatal conductance and its sensitivity to environmental conditions, is as important to branch-level sap flow as environmental gradients in the forest canopy. Soil also has major implications to transpiration rates and plant stress. Orfanus and Eitzinger (2010) demonstrated the massive impact that variability in soil properties and biological features in fields, such as hedgerows, can have on limiting the spatial distribution of water availability to crops. They adopted the concept of soil water content of limited plant availability, which encompasses transport rates in soil and evapotranspiration rates. Shadowing of soil from wind with a hedgerow was estimated to save almost 2000 m3 ha−1 of water over a growing season when evaporative demand is high. The rate of water flow under the hydraulic gradient created by plant roots needs far more consideration—plant available water ignores this limitation.
The influence of elevated atmospheric CO2 on transpiration was modelled by Anda and Dióssy (2010) for maize in the continental climate of Hungary. Similar to other studies, they demonstrated greater water use efficiency (WUE) with a doubling of CO2 if all other environmental factors remained unchanged. Coupled with increasing temperature, however, there was a decline in WUE, prompting concerns of a greater need for irrigation in the future. One method to reduce the impact of climate change and soil degradation on the need for irrigation water is an improvement in soil properties. This is one intended positive implication of adding biochar to soil, but the movement of these colloid sized particles is poorly understood. Zhang et al. (2010) studied biochar movement in column studies. Similar to other colloid studies, they demonstrate the impact of pH and salt concentrations on biochar mobility. Application of biochar close to waterways could result in significant export off-site. As the first study to consider this impact, the authors rightly point to the need for more research in this area.
More research is clearly needed in many areas of biological and hydrological interactions in soil. The authors/guest editors hope that the growing interest in this topic from hydrologists, soil scientists, biologists, ecologists, geographers and water managers will help address global challenges faced from food and water scarcity, land degradation and climate change.