ABSTRACT
The quantification of the water balance in biological systems is of fundamental importance in order to plan, allocate and manage irrigation water volumes and their quality. Data from the Food and Agricultural Organization of the United Nations (FAO) indicate that many areas of the world are affected by water scarcity coupled with population growth, competition for land and water, and projected changes in climate (FAO, 2011). Water accounting and balancing are therefore essential tools for the management of agricultural water (FAO, 2015). Efficiencies in water conveyance and distribution as well as effective rainfall need to be accounted for in gross irrigation requirements (water abstracted from a source) in order to deliver net irrigation requirements to agricultural fields (FAO-SAFR, 2002). Perry (2011) and Pereira et al. (2012) summarized the principles and terminologies
of water use, beneficial and non-beneficial water consumption, and the fate of the nonconsumed fraction. Water used (applied to agricultural fields) is partitioned into water consumed through evapotranspiration (ET) and the non-consumed fraction (excess water applied, resulting in overland flow/run-off and/or deep percolation). Water consumption can be beneficial (transpiration through plants) and non-beneficial (evaporation directly from the soil). The non-consumed fraction can be recoverable (irrigation return flow can be captured and reused by downstream water users) and non-recoverable (e.g. water that ends up in the sea). In irrigated agriculture, it is advisable to apply a leaching fraction (irrigation volume in excess of crop water requirements) to flush out salts that may build up in the soil profile, which increases the non-consumed fraction (Bresler et al., 1982). It is therefore fundamental that the components of the field water balance be determined accurately to maximize beneficial water consumption, minimize non-beneficial consumption and manage non-consumed water.
