4.5. Hydrology

Agricultural activities influence a region’s hydrology in four main ways:

(1) Plant foliage intercepts some of the rainfall that would otherwise reach the ground.

(2) When rainfall reaches the ground, it either infiltrates the soil or flows over the soil as surface runoff. Dense plant litter and porous soil increase the amount of infiltration and reduce the amount of surface runoff. Conversely, when plant litter is sparse and soil has been compacted from agricultural activities, infiltration is impeded and surface runoff increases.

(3) Growing plants absorb water through their roots and transpire it through their leaves, reducing the amount of water that recharges the groundwater supply.

(4) Crops sometimes draw upon groundwater, either directly (when their roots are sufficiently deep and the water table is sufficiently shallow - as in wetlands) or indirectly through irrigation.

Through these four mechanisms, cultivating and harvesting biomass for energy can affect the hydrological health of a region, either positively or negatively. A barren, degraded site will invariably be subject to rapid runoff and limited infiltration, minimising the rate at which groundwater is replenished. Revegetating such an area with bioenergy crops such as trees or grasses can help reduce runoff (and thereby limit soil erosion), recharge groundwater, and sustain spring-fed streams. Bioenergy crops can also be integrated into systems for harvesting surface water. Plants suitable for fuel and fodder can be planted to stabilise catchments, fortify earthen barriers and channels, and line artificial ponds. Such water harvesting techniques, combining traditional and modern technology, have been used with considerable success (Agarwal, 1997). Energy crops can also help address waterlogging problems in poorly drained or flood-prone zones.

On the other hand, fast-growing crops can consume water at excessive rates. Water transports nutrients from the roots up into the above-ground tissue, and rapidly transpires through the leaves during photosynthesis. Plants require from 300 to 1,000 tonnes of water per tonne of dry biomass (Hall et al., 1993), or 450-1500 mm per hectare per year (assuming a yield of 15 dry tonnes/ha/yr). Crops optimised for rapid growth are generally water-hungry and can be expected to consume more water than natural flora. One study noted that tree plantations "use larger quantities of water than shorter vegetation types such as scrub, herbs, and grass" and concluded that "afforestation tended to deplete substantially both the total annual water yield and the base flow in the dry season" of the affected watershed. It cautioned that "the indiscriminate planting of trees may seriously affect the viability of the springs and wetlands in many catchments" (Le Roux, 1990).

Revegetating [barren] areas with bioenergy crops such as trees or grasses can help reduce runoff (and thereby limit soil erosion), recharge groundwater, and sustain spring-fed streams.

Table 4.3. Typical Fertlizer and Herbicde Application Rates and Soil Erosion Rates for Selected Food and Energy Crop Production Systems in the United States

(a) Based on data collected in the early 1980s. New tillage practices used today may lower these values.

(b) The nitrogen input is inherently low for soybeans, a nitrogen-fixing crop.

(c) Not including nitrogen-fixing species.

Source: Hohenstein and Wright, 1994

Quick growing tree crops have been observed to adversely affect water supplies - lowering the water table, reducing stream yields, and making wells less reliable. Tree plantations in many different agro-ecological regions, including sites in Chile, India, Brazil, New Zealand, Thailand, South Africa, and Spain, have caused such impacts - one of the reasons why local agricultural communities have often opposed the introduction of tree plantations (Carrere and Lohmann, 1996). Harvesting residues, cultivating tree crops without undergrowth, and planting species that do not generate adequate amounts or types of litter, are all practices that can reduce the ability of rainfall to infiltrate soil and replenish ground water supplies, exacerbating problems of water overconsumption.

Bioenergy planners should carefully assess the potential impacts throughout a watershed of crop cultivation and harvesting, including both groundwater and surface water impacts. Species should be selected for bioenergy crops that are suited to local conditions, helping to avoid excessive water consumption and to maintain hydrological health.