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Experimental results obtained in Southern Portugal from a dry-farmed mature olive tree orchard recently converted to drip irrigation are described. Water use and response to two irrigation management practices by olive trees was monitored with sap flow compensation heat pulse sensors, ‘Watermark’ granular matrix block sensors and a capacitance probe. The 80-plus-year-old mature olive tree orchard planted on a 12m by 12m spacing layout was converted in 2005 from dry-farming to drip irrigation and subjected to two water treatments: trees irrigated daily to supply for crop water demand and trees irrigated before-flowering, during pit-hardening and before crop-harvesting. Sap flow sensors were implanted in sample trees at three different positions around the trunk and measurements were taken at 30min intervals during 4 months, from April to mid-August of 2005. Tree transpiration rates were estimated as average of sap flow rates. When trees were fully irrigated, the observed differences in daily sap flow rate amplitude were explained by the natural trees difference in canopy cover, plant height and conductance of water vapour sites. However, when deficit irrigation was prescribed and, when the trees stopped being irrigated, they gradually lost their ability to adequately respond to the evaporative demands of the day, showing smaller variations in amplitudes sap flow. After irrigation ceased in May 15, transpiration rate gradually decreased from its maximum of 7lh−1, when trees were fully irrigated and soil water content was near to field capacity, to values of less than 3lh−1 by July 3 as the soil water content gradually acted as the transpiration limiting factor. Transpiration rates recovered after irrigation was re-introduced on July 4. Although low in the non-irrigation period, transpiration rates never dropped to zero and stayed between 37 and 50ld−1 from May 27 to June 9, as trees were able to extract soil water in the absence of irrigation. Olive trees maintained transpiration to levels as high as 50ld−1 suggesting that long after irrigation is suppressed, a considerable amount of water held in the soil is made available to the trees. Differences in evapotranspiration and transpiration rates during the same period also indicated that olive trees, making use of the extensive root system developed in the 12m by 12m tree spacing, were able to extract soil water and maintain transpiration levels as high as 50ld−1, while soil water balance indicated tree evapotranspiration rates close to zero. This particular ability of dry-farmed olive trees to remove water held in the soil under adverse conditions of very low soil moisture and uncertainties associated with the real volume of soil effectively explored by the root system, make profile probe sensors, regardless of their accuracy, unsuitable for control of water uptake and management of dry-farmed olive orchards recently converted to irrigation. Likewise, watermark sensors, able to capture the variations of high soil water, failed to provide meaningfully values below 0.28cm3 cm−1. Both sensors should be used with care in managing irrigation of olive tree orchards and, rather than relying on soil moisture status, monitoring tree water-use and response with the compensation heat-pulse method seems a more appropriate approach. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]