5Soil degradation from salt accumulation, sodication, or both, is a threat or a fact in 6 many irrigated lands. Salinization has often been assessed from changing cropping 7 patterns over time, and often the trends in salinization have not been quantified. Our Thus, soil salinity in the upper meter of soil has decreased during the last 24 years.

Soil salinity encroachment is an increasing concern in many irrigated lands, because of the undesirable effects of soluble salts on agricultural production and on water quality. From this point of 10 view, the design and management of irrigation districts can be evaluated by monitoring the soil salinity.There are few cases in the world where comparisons can be undertaken from 'historic' data sets for extents other than individual plots. We demonstrate a monitoring procedure using electromagnetic induction (EMI) survey in an irrigated district in Spain. This district is the only one having an established soil salinity baseline. The EMI data acquired at the same plots were converted to soil 15 electrical conductivity by calibrating with augered soil samples. The presented calibrations improve the baseline for future comparisons and for the treatment and understanding of new acquisitions of field data in next surveys. A shortcoming inherent to destructive soil sampling is its potential for biasing effects on long-term monitoring of soil salinity by means of GPS or other means of accurate localization and relocalization of soil sampling, the herein called "localization paradox", rarely treated 20 in scientific papers. The localization paradox is relevant for any variable soil property requiring repeated sampling. This issue is discussed, and a way for its overcoming by using EMI readings displaced from the augering is presented. EMI needs calibration with a reduced number of soil samples analyzed in the lab. The adoption of our data treatment procedures will facilitate soil salinity monitoring. 25

The performance of surface renewal (SR) analysis for estimating sensible (H) and latent (λE) heat fluxes has been analysed over a sprinkler irrigated rice. Similarity principles were not fully met during the experiment and the (H+λE) measured values with the eddy covariance did not close the surface energy-balance. H and λE estimates using SR analysis were reliable and provided a reasonable energy-balance closure. The Bowen ratio was also estimated using SR analysis. Good estimates were obtained though mainly under unstable atmospheric conditions.

8Cultivation of paddy rice in semiarid areas of the world faces problems related to 9 water scarcity. This paper aims at characterizing water use in a set of paddies located 10 in the central Ebro basin of Spain using experimentation and computer simulation. A 11 commercial field with six interconnected paddies, with a total area of 5.31 ha, was 12 instrumented to measure discharge and water quality at the inflow and at the runoff 13 outlet. The soil was classified as a Typic Calcixerept, and was characterised by a mild 14 salinity (2.5 dS m -1 ) and an infiltration rate of 5.8 mm day -1 . The evolution of flow 15 depth at all paddies was recorded. Data from the 2002 rice growing season was 16 elaborated using a mass balance approach to estimate the infiltration rate and the 17 evolution of discharge between paddies. Seasonal crop evapotranspiration, estimated 18 with the surface renewal method, was 731 mm (5.1 mm day -1 ), very similar to that of 19 other summer cereals grown in the area, like corn. The irrigation input was 1,874 mm, 20 deep percolation was 830 mm and surface runoff was 372 mm. Irrigation efficiency was 21 estimated as 41%. The quality of surface runoff water was slightly degraded due to 22 evapoconcentration and to the contact with the soil. During the period 2001-2003, the 23 electrical conductivity of surface runoff water was 54% higher than that of irrigation 24 water. However the runoff water was suitable for irrigation. A mechanistic mass 25 balance model of inter-paddy water flow permitted to conclude that improvements in 26 34 976 716 145.2 irrigation efficiency can not be easily obtained in the experimental conditions. Since 27 deep percolation losses more than double surface runoff losses, a reduction in 28 irrigation discharge would not have much room for efficiency improvement. 29Simulations also showed that rice irrigation performance was not negatively affected 30 by the fluctuating inflow hydrograph. These hydrographs are typical of turnouts 31 located at the tail end of tertiary irrigation ditches. In fact, these are the sites where rice 32 has been historically cultivated in the study area, since local soils are often saline-sodic 33 and can only grow paddy rice taking advantage of the low salinity of the irrigation 34 water. The low infiltration rate characteristic of these saline-sodic soils (an 35 experimental value of 3.2 mm day -1 was obtained) combined with a reduced irrigation 36 discharge resulted in a simulated irrigation efficiency of 60%. Paddy rice irrigation 37 efficiency can attain reasonable values in the local saline-sodic soils, where the 38 infiltration rate is clearly smaller than the average daily rice evapotranspiration. 39 40 42 48 respective values of 40 and 60%. According to these figures, it is clear that paddy rice 49 ranks very low in the comparison of irrigated agricultural systems based on irrigation 50 efficiency. 51A number of authors (Keller et al., 1996; Perry, 1999) have emphasized the hydrologic 52 implications of irrigation efficiency, partic...