Francisco L. Santos

Comparative Assessment of Infiltration, Runoff and Erosion of Sprinkler Irrigated Soils

Abstract The impacts of sprinkler irrigation on infiltration, runoff and sediment loss of ten representative soils of Southern Portugal were assessed by laboratory sprinkler irrigation simulation tests. All soils showed very low permeability to applied water. The mechanical impact of water droplets enhanced soil dispersion and further lowered their infiltration capacity, particularly for high clay plus silt content soils that showed the poorest results. As a consequence, high runoff and sediment losses were also measured, primarily with the first irrigation. More moderate losses were observed thereafter. Soils with higher sand particle size fractions better absorbed the energy impact of droplets and showed higher infiltration rates and lower runoff and sediment losses. Polyacrylamide (PAM) applied to the soils through the irrigation water acted as a binding and settling agent to increase soils aggregate stability and infiltration and reduce runoff and sediment losses. Slope increase, from 2·5 to 5%, decreased overall soils infiltration by 7% and increased runoff and sediment losses by 10 and 27%, respectively. Exposed to the same change in slope, PAM application boosted overall infiltration of treated soils to a 24% difference and increased runoff by only 10%. It had a less positive effect on sediment loss, the 5% slope being responsible for a 52% increase. In agreement with this the tests showed that, compared to the control, exposure of PAM-treated soil on 2·5 and 5% slopes enhanced overall infiltration to 457 and 642% respectively, reduced runoff by 25% on both cases and lessened sediment loss by 39 and 27%. The demonstrated ability of PAM to influence surface soil conditions of specific soils can be used to reduce the environmental risks associated with the intensive use of sprinkler irrigation in Southern Portugal. It offers a safe, practical and non-intrusive management alternative to current costly, labour- and energy-intensive practices of increasing the number of machine turns and building storage basins to control runoff and soil erosion.

Hargreaves and Other Reduced-Set Methods for Calculating Evapotranspiration

Globally, irrigation is the main user of fresh water, and with the growing scarcity of this essential natural resource, it is becoming increasingly important to maximize efficiency of water usage. This implies proper management of irrigation and control of application depths in order to apply water effectively according to crop needs. Daily calculation of the Reference Potential Evapotranspiration (ETo) is an important tool in determining the water needs of different crops. The United Nations Food and Agriculture Organization (FAO) has adopted the Penman-Monteith method as a global standard for estimating ETo from four meteorological data (temperature, wind speed, radiation and relative humidity), with details presented in the Irrigation and Drainage Paper no. 56 (Allen et al., 1998), referred to hereafter as PM:

Quality and maximum profit of industrial tomato as affected by distribution uniformity of drip irrigation system

The tomato industry reformed its system of payment by weight of tomato, introducing a corrective system based on percent level of fruit dry matter produced. Such a decision implies significant changes in the management of irrigation systems, with a need to emphasize the technological quality of the marketable product. Three levels of distribution uniformity of the irrigation system are analysed, and related production functions of crop yield and percent of dry matter are presented as well as their use on the optimisation of dry matter, expected revenues and seasonal applied water. Results are critically influenced by the distribution uniformity. They demonstrate the inter-relationship between crop production, percent fruit dry matter and irrigation management, and the importance of considering non-uniformity in the economic analysis of industrial tomato production. Decreases in uniformity lead to a reduction in dry matter production per unit land. Decreases in dry matter are also observed with increasing levels of seasonally applied water, with the optimal level always lower than the required for maximum yield. Such interaction suggests a continuous and inverse relationship between profit and water applied. However, due to the corrective system of payment, by levels of percent of dry matter produced, for some uniformity, the expected revenue follows the yield-water production function instead of the dry matter function. This fact introduces disturbances in the optimal water applied inducing higher than expected levels of water applied for profit maximisation. The simulated data also show that incentives to switch to new systems or management practices able to raise the distribution uniformity result more from profit losses than increases in water price.

Center Pivot Sprinkler Irrigation Infiltration, Runoff and Erosion Control with Polyacrylamide

Polyacrylamide (PAM) applied to furrow irrigated fields at rates of about 1 kg per hectare has been reported to enhance infiltration and control irrigation-induced erosion (Santos and Serralheiro, 2000). Under furrow irrigation PAM dissolved in the furrow stream contacts the soil surface and stabilizes soil structure against shear-induced detachment by enhancing particle cohesion, thereby preventing transport in runoff. Based on the success of PAM with furrow irrigation, many irrigators are interested in using PAM with sprinkler irrigation. Differently from furrow irrigation, water drop impact energy and splash break down soil aggregates and crust soil surface, leading to surface sealing of sprinkled soil surface, and the subsequent runoff and erosion-induced sediment loss. We applied i) 1 kg PAM per hectare to two Fluvisols prior to irrigation and ii) 1 kg ha -1 PAM in the pumped irrigation water to a Luvisol, to identify the effectiveness of PAM in controlling infiltration, runoff and soil erosion of these soils when irrigated with a center pivot system. Field results indicate that significant, and long lasting, infiltration enhancement and runoff and erosion control can be achieved in all studied soils with a single application of 1 kg PAM ha -1 . In the silty-loam Fluvisol PAM treatment reduced erosion approximately 64 and 76% in fallow and crop growing plots, and enhanced infiltration in 34 and 18%, respectively, when compared to control. In the sandy-loam Fluvisol the results, when compared to control, were more dramatic with PAM application reducing soil erosion 98 and 96% in the fallow and crop growing plots and enhancing infiltration in 47 and 45 %, respectively. Concerning the Luvisol, PAM treatment reduced erosion in 46% and enhanced infiltration in 15% in the fallow plots, when compared to control. No significant differences were observed between fallow and crop growing treatments. The field results with PAM applied to Luvisols were significant but less conspicuous than with Fluvisols, due probably to their lesser amount of silt and clay in the soil surface, which not lead to considerable erosion, crusting and surface soil sealing to induce runoff and prevent soil infiltration. Preliminary laboratory tests used to scrutiny PAM effects on soil aggregate stability and infiltration enhancement were important but not sufficiently consistent in all cases to lead to predictions of PAM sediment loss and infiltration control when applied to the fields through center pivot main line.

Evaluation and adoption of irrigation technologies. I. Management-design curves for furrow and level basin systems

Abstract The effect of average low-quarter requirement depth of irrigation and uniformity of applied water on the estimation of inflow rate and cutoff time of furrows and level basins is evaluated, using the SRFR-surface irrigation model and different intake rate soil characteristics. The optimum inflow rate and cutoff time is dependent on the irrigation technology and infiltration characteristics. To assist in the task of on-farm water management of furrows and level basin irrigation and the engineering design and planning of these systems management, design charts are proposed that define the optimum combination of inflow rate and cutoff time, given values of required depth and uniformity of application. The charts are plots of average low-quarter requirement depth and uniformity contours on axes of inflow rate and cutoff time. The general form of the curves shows that technical trade-off between systems becomes evident as flow rates and cutoff times decline as fields are levelled to zero slope and irrigated as basins. The relative steepness and the dense nature of the level basin uniformity contour curves reflect the importance of intense management and the greater penalty for management errors with these systems as opposed to furrows.