The objective of this paper was to assess the performance of Hydrus-2D model to simulate the effects of different on-farm irrigation strategies applied on potato crop. The ability of the model to simulate the stress coefficient (Ks), obtained as the ratio between actual and maximum transpiration, and to define the productive function of potato crop under the semi-arid conditions of central Tunisia were also evaluated. Experiments were carried out on potato crop under full (FI) and deficit irrigation (DI) and two different water qualities supplied by means of a subsurface drip irrigation system. Results evidenced that the model, despite some discrepancies locally observed, can fairly accurately predict soil water contents and electrical conductivity around buried emitters. Furthermore, under water and salt stress conditions, “measured” Ks, based on crop water stress index (CWSI) obtained on thermal images, resulted in a good correlation with the corresponding estimated by the model (R2 = 0.8). The database collected during the three growth seasons also allowed the definition of the crop productive function represented by a linear relationship between the relative yield loss and Ks. This function represents a useful guidelines for the sustainable use of irrigation water in countries characterized by a semi-arid climate and a limited availability of water for irrigation.
Water supplies have been decreasing in several semi-arid regions, and it is therefore necessary to adopt irrigation strategies aimed at maximizing water use efficiency. In this paper, the effects of saline and deficit irrigation on water use efficiency and on potato crop response, based on observations of soil and plant water status, were investigated. Experiments were carried out in Central Tunisia, by monitoring potato crop growth during two seasons in four distinct treatments (T1–T4), represented by two different irrigation doses and two water qualities. For irrigation scheduling purposes, thresholds of soil matric potential, soil water content and Crop Water Stress Index (CWSI) were identified with the aim to quantify the effects of water and/or salinity stress on the achievable yield. Experiments allowed verifying that crop yield is strongly affected by the seasonal amount and quality of applied water. Despite differences of crop yield between treatments T2, T3 and T4 not being statistically significant (P < 0.05), crop yield varied between 26.3 t/ha (T3 in 2015) to 16.3 t/ha (T4 in 2015). However, crop yield decline of 17.0 t/ha and 12.0 t/ha per each 100 mm decrease of applied water were observed under the application of water electrical conductivity of 1.6 dS/m and 4.1 dS/m respectively. On the other hand, an increase of 1.0 dS/m in water electrical conductivity caused a yield decline rate of about 10%. The results achieved showed that under the semi-arid climate of Tunisia, potato crop irrigation should be scheduled to avoid water deficit; however, the possibility to reduce water supply can be envisaged when water availability is limited, but with the awareness to accept the shortage of production. Finally, when saline water is the only source available to the farm, it is necessary to avoid the reduction of irrigation doses, to prevent excessive salt accumulation in the root zone with unavoidable effects on crop yield.
Under semi-arid conditions, irrigated agriculture faces hard competition for water. It is against this backdrop that appropriate management of irrigation techniques and water resources becomes a major concern. This study investigated the effect of surface (SDI) and subsurface drip irrigation (SSDI) with domestic treated wastewater (TWW) and fresh water (FW) on soil water dynamics, salinity, yield, and mineral nutrition of okra. The experimental design was set-up based on two adjacent plots according to the water quality: Fresh Water (FW) T1 and domestic Treated Wastewater (TWW) T2. Results showed that measured soil water contents (SWCs), under TWW treatment (T2), were greater than their corresponding measurements under FW (T1), and in particular at 35 cm depth. Meanwhile, for both water qualities, soil Electrical Conductivity (EC) registered at 5 cm depth was higher than those measured at 35 cm, with values ranging from 0.14 to 0.36 mS·cm−1 and from 0.20 to 0.47 mS·cm−m for T1 and T2, respectively. Regarding crop yield, a statistically significant increase (p = 0.05) in okra fresh yield was observed when TWW was used. Fresh yield in SDI was 2.55 t·ha−1 and 3.9 t·ha−1 in T1 and T2, respectively. Nevertheless, results indicated that lateral depth did not significantly affect okra fresh yield. Moreover, a significant higher irrigation water productivity (WPirrig) with TWW (1.08 ± 0.26 and 1.23 ± 0.18 kg m−1) was observed, which was nearly double those obtained with FW (0.72 ± 0.33 to 0.78 ± 0.18 kg m−1). Appropriate use of SSDI with TWW stands as an irrigation management technique to improve yield and irrigation water productivity of okra crops.
In Tunisia, water scarcity forces producers to face stress conditions. In this study, AquaCrop was used to reproduce the dynamic of water contents, vegetative growth, yield production and water use efficiency under a non-stressed and water stressed treatments. Calibration procedure aimed to use in maximum default parameters of AquaCrop. Since, the paper presented only the parameters that have to be adjusted to obtain similar results of field measurements. Root mean squared error, RMSE, values were always lower than 0.04 cm3.cm-3 for water contents lower than 0.06 for vegetation cover estimation. Moreover, results from Nasch Coefficient, E, were almost equal to one. RMSE and E justified that the model was well assessed to predict the soil water contents and vegetation development under the study area. However, the model presented a greater performance in the case of full irrigation strategy. When evaluating different values of water productivity, it was showed that a WP of 32 g.m-2 produced the lowest estimation error. Regarding yield productions, statistical indictors, computed for a water productivity value of 32 g.m-2 show in general RMSE values lower than 0.4 t/ha. In addition, E was closer to one for the non stressed treatment, T1. For irrigation water use efficiency, it was depicted that the model underestimated field IWUE. Moreover, the discrepancy between simulated and estimated irrigation water use efficiency rose for treatment T2, implying that the model calibration should be improved, especially for stressed conditions. The model, after being calibrated, could be used for simulating the response of the crop to different irrigation management aiming to optimize water use efficiency.
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