Castilla y Leon, with 94 000 km2, is the largest Spanish Autonomic Community and one of the major administrative regions of Europe. Castilla y Leon is basically a large plateau, with an important agricultural production. The region involves more than 20% of the cultivated area of Spain, 22.7% of grassland area and 31.9% of the cereals-cropping area. Even though Castilla and Leon comprises only 6% of Spanish population, they mean about 12% of the reported farm-workers in Spain, almost twice the country mean (7.7%) and higher than the European mean (9.0%).
Castilla y León is, after Andalusia, the Spanish Autonomic Community with higher irrigation surface. According to the last Ministry of Agriculture report (MAPA, 2004), Castilla y León irrigated-agriculture produces 57% of Sugarbeet, 38% of maize and 30% of potatoes of the whole country. The investment in irrigation infrastructures in Castilla y León is the second highest in Spain. However, such investment must be joined to applied researches aimed to improve water-management efficiency, particularly in eventual prices diminishing or severe droughts.
The recent reform of the European Sugar Market (EC Council Regulation 318/2006) will bring important reductions on the Sugarbeet prices. Irrigation means about 35% of the total Sugarbeet production costs in Northern Spain. Sugarbeet water requirements will be higher in the future, according to Climate-Change predictions, while water availability will diminish in the Castilla y León zone. Furthermore, EU Water Framework Directive (EC Directive 2000/60), through its "recovering costs" principle, will very probably increment the water prices. The combination of all these factors could yield that Sugarbeet becomes an unaffordable crop in Castilla y León in the near future.
According to the above, an AGRIDEMA Pilot assessment was conducted in order to calibrate and validate the SWAP model for Sugarbeet water-use simulations, as well as to estimate Climate Change effects on water use efficiency, considering a typical irrigation management. Utset et al. (2007b) provided the calibration and validation of the SWAP model. The assessment was conducted at Valladolid, Northern Spain (41°39'N, 4°43'W). The regional climate is Mediterranean Semiarid with an annual average precipitation of 531 mm. According to the local soil map (JCYL, 1987), Sugarbeet is cropped mainly in Cambisols and Fluvisols.
4.5.1 SWAP Calibration and Validation. The Assessment Data
The SWAP calibration comprised data of experiments addressed to study water-shortage effects on sugarbeet growth, made during 1992, 1993, 1994 and 1995 in a clayey Fluvisol and in a Cambisol (Velicia, 1998). The water-stress effect on sugarbeet yields depends on the growing period (Brown et al., 1987; Groves and Bailey, 1994; Fabeiro et al., 2003). Accordingly, water shortages were applied at the beginning and end of sugarbeet development. Four irrigation start and end dates were considered (Velicia, 1998), based on non-restrictive irrigation management with typical considerations (Morillo, 1993; Allen et al., 1998), which provides all the sugarbeet water requirements from seeding to harvest. Four plots were dedicated to each irrigation design at each soil type. The plots were separated from each other by at least 20 m in other to avoid water redistribution among them.
Sugarbeet roots can reach up to 2 m in length (Brown et al., 1987; Velicia, 1998). The root distribution in depth depends on soil and moisture conditions. However, according to Velicia (1998), about 60% of the roots is usually found in the first 30 cm, whereas 90% could be found at depths shallower than 90 cm. Therefore, a non-linear root distribution was assumed.
Potential evapotranspiration was calculated by the Penman-Monteith approach, using meteorological data available from stations at distances of 1.2 and 2.3 km from the plots. Maximum crop evapotranspiration was calculated considering the Kc coefficients estimated for the zone (Morillo, 1993). Daily temperature values were also recorded. The SWAP FCS stage was considered as the date when sugarbeet covers soil completely under non-restrictive irrigation water-management conditions (Velicia, 1998). The maturity date was assumed as the harvest.
The sugarbeet (Ramona cultivar) germination date was May 15 and the average harvest date was October 15. The said dates were considered for computing the temperature sums at FCS and maturity and can be considered as typical dates for spring-sown sugarbeet (Morillo, 1993). The irrigation water applied at each plot was measured weekly. Final sugarbeet yields were measured at each plot.
The leaf area index (LAI) was measured monthly at each plot. The length and width of all the green leaves on each plant in 1-m rows were measured. LAI was indirectly estimated according to a regression equation obtained from previous studies (Velicia, 1998). Root lengths and weights were also measured (Velicia, 1998) from three randomly selected plants, removed entirely from the ground at each plot on a monthly basis. Soil water content was measured weekly by a neutron probe (Velicia, 1998). The actual crop evapotranspiration was estimated by water balance (Allen et al., 1998).
The actual crop transpiration was separated from total evapotranspiration considering the experimentally-measured LAI values (Ritchie, 1998). Final yields were measured and total crop transpiration was computed as the cumulative sum; the Ky coefficients (Doorenbos and Kassam, 1979; Kroes and Van Dam, 2003) were then calculated. The hydraulic properties of the soil were estimated from the physical soil data taken at each plot (Velicia, 1998) through a pedotransfer function (Shaap et al., 2001). SWAP simulations were performed at each plot, considering the parameters obtained in the calibration and free drainage at the bottom of the 1-m soil layer. A regression between relative yields and actual yields was conducted in order to find which maximum yield adapted better to the experimental conditions.
An independent data set was used for SWAP validations. Two sugarbeet plots, corresponding to a Cambisol (Plot Z) and a Fluvisol (Plot A), were selected in the Valladolid area. The plots were separated from each other by a distance of approximately 15 km.
Sugarbeet was seeded in the two plots in March 2005. Emergence dates were April 5 (A) and April 25 (Z). Undisturbed soil samples were taken at 20-40 cm depth in ten randomly-selected sites within each plot. The soil water retention curves were determined at each sample through a sand-kaolin box and a Richard membrane (Klute, 1986). The saturated hydraulic conductivity of each sample of soil was measured in a laboratory permeameter. The soil field capacity and the wilting point were considered as the water contents at 33 and 1500 kPa, respectively. The parameters of the Van Genuchten model for the soil-water characteristic curve were estimated through the RETC code (Van Genuchten et al., 1991).
Access tubes were placed at the same places where soil samples were taken at each plot. Soil water contents were measured weekly during July and August 2005 at 0-20, 20-40 and
40-60 cm depths by a Trime TDR [IMKO Micromodultechnick GmbH]. Actual crop evapotranspiration was computed using the balance method from the measured water contents and neglecting any possible capillary rising, runoff and/or percolation. Daily precipitation, global radiation, maximum and minimum temperature, wind speed and relative humidity values were recorded at two agrometeorological stations located near the selected plots.
Irrigation was applied traditionally in a way similar to that used by farmers to manage sugarbeet water applications at each plot. The irrigation water supply at each plot was measured through a water counter. Sixteen irrigations were applied on the Z Plot from June to August, comprising a total water application of 508 mm. Likewise, 556 mm of water were applied by irrigation on the A plot in the same period over eighteen irrigations. Precipitations are scarce in the Mediterranean summer. Accordingly, the total rainfall recorded was 41 mm on the Z Plot and 25 mm on the A Plot during the 2005 sugarbeet cropping season.
Actual SWAP simulations were conducted using the measured soil and meteorological data and considering the model calibration performed. Free drainage at the bottom of the 1-m simulated soil layer was considered, which is consistent with the soil descriptions (JCYL, 1987). Correlation and determination coefficients between simulated and estimated actual evapotranspirations were calculated, as well as the same coefficients between the SWAP-simulated actual evapotranspirations and the sugarbeet water requirements, calculated from Penman-Monteith computations of the reference evapotranspiration and the sugarbeet coefficients in use, as provided by Morillo (1993). Furthermore, Root Mean Square Errors (RMSE) were calculated between the simulated ETC and the water-balance estimated ETC, as well as between the simulated ETC and the maximum ETC, estimated from weather data. The RMSE has been considered as the most effective method of comparing simulated and actual values in evaluation model performance (Willmott et al., 1985; Timsina and Humphreys, 2006). The normalized RMSEn is used to compare modelling performances and can be calculated as:
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