1991). IWM systems are typically knowledge intensive and make use of ecological principals. Examples of IWM elements include cultivars that are bred for competitive ability (Gibson et al., 2003), diverse crop rotations that provide a variety of selection and mortality factors (Westerman et al., 2005), and simple management changes like higher seeding rates, spatially-uniform crop establishment (Olsen et al., 2005), banded fertilizer placement (Blackshaw et al., 2004), and biological control, particularly when the weed is an exotic invader (Zimmermann and Olckers, 2003). The serious parasitic weed of cereal crops (Striga spp.) in Africa can be regulated in sorghum by varietal resistance (Tesso et al., 2006), and by bait crops, like Sesbania sesban, Desmodium spp. that trigger suicidal germination of Striga seed (Gatsby Charitable Foundation, 2005; Khan et al., 2007). Herbicide use in agriculture has not been markedly reduced by integrated weed management, as weed science has lagged behind pest and disease management initiatives in terms of developing the basic biological and ecological insights typically required for integrated management (Mortensen et al., 2000; Nazarko et al., 2005).

3.2.2.1.3 Integrated water resources management (IWRM) IWRM acknowledges water resource management conflicts by using participatory approaches to water use and management; resource development and environmental protection (van Hofwegen and Jaspers, 1999). It recognizes that water use in agriculture, especially irrigation water, meets the needs of fisheries, livestock, small-scale industry and the domestic needs of people, while supporting ecosystem services (Bakker et al., 1999; CA, 2007).

IWRM helps to resolve the numerous conflicts associated with water use and management; resource development and environmental protection.

Examples of IWRM at the field scale include alternate tillage practices to conserve water and low-cost technologies such as treadle pumps (Shah et al., 2000), and water-harvesting structures. IWRM recognizes the need to integrate water management at the basin level and to promote the linkages between different water uses at this level. It supports river basin management to ensure optimal (and efficient) allocation of water between different sectors and users. Through these approaches, IWRM has achieved a better balance between protecting the water resources, meeting the social needs of users and promoting economic development (Visscher et al., 1999).

Natural Sequence Farming is restoring the hydrological balance of dryland farms in Australia.

Many agricultural landscapes in Australia are facing a land degradation crisis as a result of increasing salinity, soil acidification and erosion, coupled with severe drought, costing the economy 2.4 billion year--1 (CRC Soil and Land Managechapter

ment 1999; Boulton, 1999, 2003). Much of this degradation has been caused by land clearance, clearance of waterways, and inappropriate European farming methods (Erskine, 1999; Erskine and Webb, 2003). Natural Sequence Farming is based on an understanding of how water functions in and hydrates the floodplain and involves techniques to slow down the drainage of water from the landscape and reinstate more natural hydrological processes (Andrews, 2005). The reported impacts (www.naturalsequence farming.com) of this have included increased surface and subsurface water storage, reduced dependence on borehole water from aquifers, significantly reduced salinity, improved productive land capacity, recharged aquifers, increased water use efficiency, increased farm productivity with lower water inputs, reduced runoff during peak inflows, and reduced use of pesticides (85%), fertilizers (20%) and herbicides (30%).

Forestry has a role in regulating water supplies for agriculture and urban areas.

The deforestation of watersheds has led to flooding; landslides; downstream siltation of waterways, wetlands and reefs and water shortages. However, the role of forests in regulating the availability of water resources involves a complex set of relationships involving site-specific functions of slope, soil type and surface cover, associated infrastructure and drainage, groundwater regimes, and rainfall frequency and intensity (Calder, 2005). Water quality from forest catchments is well recognized as better than that from most alternative land uses (Hamilton and King, 1983; Calder, 2005). In spite of the lack of clarity of land use-hydrological relations, payment systems or markets for watershed services are becoming popular in urban areas. For example, New York City has been assisting farmers to change land use, and in doing so has avoided the cost of constructing a large water purification plant.

3.2.2.1.4 Integrated soil and nutrient management (ISNM) There are multiple pathways for loss of soil nutrients from agroecosystems, including crop harvest, erosion, and leaching. Soil nutrient depletion is one of the greatest challenges affecting the sustainability and productivity of small-scale farms, especially in sub-Saharan Africa. Globally, N, P and K deficits per hectare per year have been estimated at an average rate of 18.7, 5.1, and 38.8 kg, respectively (Lal et al., 2005). In 2000, NPK deficits occurred respectively on 59%, 85%, and 90% of harvested area. Total annual nutrient deficit (in millions of tonnes) was 5.5 N, 2.3 P, and 12.2 K; this was associated with a total potential global production loss of 1,136 million tonnes yr-1 (Lal et al., 2005). Methods for restoring soil fertility range from increased fertilizer use to application of organic amendments like compost or manure. Applied in sufficient and balanced quantities, soil amendments may also directly and indirectly increase soil organic matter (see also 3.2.1.5). In addition to providing a source of plant nutrition, soil organic matter can improve the environment for plant growth by improving soil structure. A well-structured soil typically improves gas exchange,