Parallel Sesion 5: Environmental Impact
Parallel 5.1
AN INTEGRATED MODELLING APPROACH FOR IRRIGATION WATER MANAGEMENT USING SALINE AND NON-SALINE WATER: THE SALTMED MODEL
R RAGAB
Centre for Ecology and Hydrology, CEH, Wallingford, OXON, OX10, 8 BB, UK
E-mail: rag@ceh.ac.uk
A successful water management scheme for irrigated crops requires an integrated approach that accounts for water, plant, soil and field management. For that purpose, the SALTMED model has been developed. The model runs on a PC under Windows 95/98 operation System. The model's input consists of: Climate data, Soils data, crop data, irrigation data (System, amount, salinity), soil parameters, crop parameters, and other model parameters. The model has default values and includes database for soils and crops. In the model, the Richards Equation and the Convection-Dispersion Equation describe the water and solute movements respectively. The daily potential and actual evapotranspiration were calculated using Penman-Monteith equation according to FAO Irrigation & Drainage paper No. 56. The model runs for a variety of irrigation systems, crops, soils, and water salinity levels. The daily model output ( graphs and data files) includes, yield, potential and actual water uptake, salinity, soil matric potential and soil moisture profiles, crop water requirements, leaching requirements, plant growth parameters, Potential and actual evapotranspiration, bare soil evaporation and plant transpiration. The model is friendly and easy to use benefiting from the windows environment.
Parallel 5.2
SUSTAINABILITY OF HIGH FREQUENCY IRRIGATION WITH GYPSIFEROUS MINE WATER
N Z JOVANOVIC1, J G ANNANDALE1, P D TANNER2 and N BENADÉ3
1Department of Plant Production and Soil Science, University of Pretoria, Pretoria, 0001 South Africa
E-mail: annan@scientia.up.ac.za
2Anglo Coal Environmental Services, P O Box X9, Leraatsfontein 1038, South Africa
3NB Systems, P O Box 15102, Sinoville 0129, South Africa
The use of gypsiferous mine water for irrigation of agricultural crops is a promising technology which could turn an environmental problem into a valuable resource. Precipitating gypsum in the soil profile could remove a considerable amount of salts from the water system, without causing irreparable damage to soil resources. The objective of this work was to recommend sustainable cropping and irrigation management strategies, by making use of a simple, mechanistic, generic crop model (SWB, Soil Water Balance). The SWB model, in combination with the CLIMGEN weather data generator, was used to quantify the long-term water and salt balance for different cropping systems, irrigation water management, water qualities and climates. For the purpose of effluent mine drainage utilization, it is important to have as large a transpiring canopy as possible throughout the year. Fast-growing species that use a lot of water are recommended (maize in combination with a winter cereal, or a lucerne-fescue mixed pasture). High frequency irrigation, leaving room for rain, is recommended to minimize salt leaching. In the presence of Mg2+ or Na+ in gypsiferous irrigation water, more salt leaching and less favourable conditions for crop growth can be expected, compared to water qualities that resemble a saturated gypsum solution. A leaching fraction is recommended for salt sensitive crop species irrigated with gypsum water rich in Mg2+ and Na+, especially during dry periods. According to the simulation results, more gypsum is expected to precipitate when the atmospheric evaporative demand is high (hot and dry conditions), compared to cool and humid climates. Proper fertilization, agronomic and soil amelioration practices must be ensured, and the economic feasibility of cropping and irrigation management systems should be investigated.
Parallel 5.3
WATER MANAGEMENT FOR CONTROLLING ANOPHELES MOSQUITOES IN RICE FIELDS
C M MUTERO1,2, H BLANK2, F KONRADSEN2 and W VAN DER HOEK2
1International Centre of Insect Physiology and Ecology (ICIPE), P O Box 30772, Nairobi, Kenya
E-mail: cmutero@icipe.org
2International Water Management Institute (IWMI), P O Box 2075, Colombo, Sri Lanka
An experiment to assess the impact of intermittent irrigation on Anopheles larval populations and rice yields was conducted in the Mwea rice irrigation scheme in Kenya. Four water regimes including intermittent irrigation were tested in a complete randomized block experimental design. Intermittent irrigation was carried out on a weekly schedule, with flooded conditions from Saturday through Tuesday morning. Larval sampling at each plot was conducted every Monday and prior to draining of intermittently irrigated subplots on Tuesday. All the adult anopheline mosquitoes emerging from larvae collected in the experimental plots were identified as being An. arabiensis. By far the highest numbers of An. arabiensis 1st instar larvae were found in the intermittently irrigated subplots, indicating that the water regime provided the most attractive environment for egg laying. However, the ratio between the 4th and 1st instar larvae in the subplots was only 0.08, indicating very low survival rates. In contrast, the 4th/1st instar ratio for subplots with other water management regimes ranged between 0.27 and 0.68, suggesting a correspondingly higher survival than observed with intermittent irrigation. The total number of 4th instars was nevertheless almost the same in the intermittently irrigated subplots and the irrigation system normally practiced by the farmers. The failure to eliminate larval development up to the 4th instar in the former method was attributed to residual pools of water. Larval abundance fluctuated throughout a 12-week sampling period. The highest larval densities were recorded in the three weeks after transplanting the rice seedlings. Afterwards, larval numbers dropped dramatically as the height of rice plants increased. Rice yields at harvest did not show statistically significant differences among subplots with different water regimes. The average yield per hectare ranged from 4.8 – 5.3 metric tons. In view of the encouraging results with regard to larval mortality, further research is necessary to among other things determine whether rice yields could be increased by having flooded and drained intervals that were different from those used in the study. It would likewise be important to assess on a wider scale the feasibility of implementing intermittent irrigation with respect to farmer acceptance and required changes in irrigation system design and management.
Parallel 5.4
OPTIMIZATION OF THE USE OF SALINE IRRIGATION WATER FOR APRICOT TREES
T VOLSCHENK1 and J DE VILLIERS
1ARC Infruitec-Nietvoorbij, P/B X5029, Stellenbosch, 7599 South Africa
Approximately 47% of apricot tree plantings in South Africa are centered on Montagu and Barrydale in the Little Karoo. Below average production in this area could be ascribed to the deteriorating water quality the Breede River and highly saline groundwater from boreholes which provides this area with irrigation water. Profit margins for farmers are such that decreased yields cannot be tolerated. Correct management of low quality water could improve production and net farm income and could decrease return flow into the river system. The objective of this work was to establish whether international water quality guidelines for apricot are applicable under local conditions and to provide guidelines for the management of irrigation with saline water. A drainage lysimeter was used to evaluate the effect of saline irrigation on apricot (armeniaca cultivar Palsteyn) trees over a period of four years at Stellenbosch (S33o 55'; E018o 53') in the Western Cape. Salinity levels included a control (municipal water) and target levels of 0.7, 1.0, 2.0, 3.0 and 4.0 dS.m-1. Saline solutions were obtained by mixing different volumes of a CaCl2:NaCl (1:1 molar) stock solution with control treatment water. The effect of saline irrigation water on soil water salinity, vegetative growth, reproductive growth and the physiology of the trees were monitored. Leaf area duration decreased with increasing irrigation water salinity. Leaf water potential, leaf osmotic potential and relative water content of leaves decreased significantly with increased irrigation water salinity. The reduced canopy area in the higher salinity irrigation water treatments intercepted less light and, in combination with lower stomatal conductance and decreased net photosynthesis rate of leaves, led to reduced water consumption, tree trunk growth and final fruit size. Irrigation water salinity levels of 1.0 dS m-1 or higher, with a leaching fraction of 0.1 applied, led to soil water concentrations that exceeded the locally-determined threshold salinity value of 3.0 dS m-1 for potential growth and yield decrement. This value is similar to the internationally recommended value of 3.2 dS m-1. Growers were therefore advised not to use irrigation water with salinity in excess of an electrical conductivity of 0.71 dS m-1 for the irrigation of Palsteyn apricot on Marianna rootstock where a leaching fraction of 0.1 is applied. The irrigation water salinity that could be used without yield loss at leaching fractions of 0.15 to 0.20 was estimated as 0.94 dS m-1 and 1.15 dS m-1. During this work the assumption was made that salts were not leached out by rainfall.
