Abstract: content as an attempt to monitor the irrigation time and as an approach for soil workability using the simple measure of the water table depth.
4- quantify the effect of the soil water table depth, through the distance from the drain, on the crop yield and water use efficiency, under different drainage conditions.
To achieve the aim of this study, two sites were selected to represent the different drainage conditions, i.e. adequate (site 1) and inadequate (site 2) subsurface drainage system. The selected two sites located at Ebiar village in Kafr El-Zayat district, El-Gharbia Governorate. At each site, observation wells were installed along a line perpendicular of three laterals. Observation wells were located at a distance of 1/8, 1/4 and 1/2 the drain spacing (40 m). The two sites were cultivated, in agricultural cycle, with wheat and corn crops during the two successive growing seasons of 1998/1999 and 1999/2000. The experiment was arranged in split split plot design with three replicates. The main plot was assigned as the drainage status; adequate and inadequate drainage. The subplot was the distance from drain (lateral) which were 5, 10 and 20 m. The sub-subplot was assigned as the plant growth stages. Each plot was 2m×2m (4m2).
In each site disturbed and undisturbed soil samples were collected from soil profiles every 30 cm depth until 60 cm. Each site was represented by two profiles. One profile above the drain pipe and the other in the midway between each two laterals for comparison. The soil samples were used to determine the main physical and chemical soil properties.
Water table depth was recorded daily between every two irrigations cycle during the growing seasons of wheat and corn crops. Rate of water table drawdown, the frequency distribution of water table and the relative ground water depth were calculated to evaluate the ground water table and the performance of the drainage system. Also drain discharge was measured during all irrigation cycles and some hydrological parameters such as drainable intensity factor and the effective porosity were calculated. The hydraulic conductivity and the pore size distribution were also determined.
The measurements of the soil water table depth (WTD) were made simultaneously with the soil moisture content (MC) at 5, 10 and 20 m distance from drain during the growing seasons of wheat and corn for each study site. Also the soil moisture content at the lower plastic limit (LPL) for the surface soil layer (0-30 cm depth) was determined.
Finally, some vegetative growth parameters, yield and yield components, water consumptive use (Cu) and water use efficiency (WUE) of wheat and corn crops were determined for the two sites along the two years study period.
The obtained results can be summarized in the following:
1. Evaluation parameters of the WTD fluctuation, rate of drawdown, relative depth and the frequency distribution of the WTD, depend mainly on the drainage conditions and the distance between the drains. Consequently, these parameters are good indicator to evaluate the efficiency of the subsurface drainage (tile drainage) average values of each of the studied parameters of the ETD were greater under the adequate drainage conditions than that under the inadequate one.
2. Higher WTD due to the inadequate drainage conditions, resulted in low values of the studied hydrophysical properties of the soil. Average values of drainage intensity factor "a", saturated hydraulic conductivity "K", effective porosity "f", quickly drainable pore "QDP" and slowly drainable pore "SDP" were about 2.2 and 2.0, 1.26 and 1.4, 2.1, 1.4 and 1.6 times greater under the adequate drainage conditions than that under the inadequate one. Whereas average value of WHP and FCP were, respectively, lower 0.92 and 0.87 times under adequate drainage conditions than the inadequate one. Average values of these soil properties were, also, found to be decrease with increasing the distance from drains to the mid-distance between them.
3. The poor conditions for plant growth resulting from the inadequate drainage conditions are responsible for average crop production losses equal to 31.6 and 43.2% of the production under the adequate drainage conditions, respectively, for grain yield of wheat and corn crop. The average grain yield for the two seasons of study was 2.59 and 3.49 ton/fed., respectively, for wheat and corn crops under the adequate drainage conditions. While it were 1.77 and 1.98 ton/fed., respectively, for the same two crops.
Also the undesirable soil properties due to the higher WTD obtained at 20 m distance from the drains, are responsible for grain yield losses equal to 29.1 and 26.7% of the yield obtained at 5 m distance from drains where WTD is always more deeper relative to that at mid-distance from drains, respectively, for grain yield of wheat and corn. The average grain yield of the two seasons was 2.51 and 3.14 ton/fed., respectively, for wheat and corn at 5 m distance from drains. While it was 1.78 and 2.3 ton/fed., respectively, for the same two crops at 20 m distance from the drains.
4- The seasonal water Cu of wheat and corn under the adequate conditions was about 15.6 and 16.7% more than that under the inadequate one. Values of the Cu decreases as WTD decrease as well as that of the WUE. The WUE of wheat and corn grains were, respectively, 1.42 and 1.5 times more under adequate drainage conditions than that under the inadequate one.
5- The predicted W.T.D at the soil MC, in the root zone depth, which meets the irrigation time could be used as a new approach fast and easy for irrigation timing of wheat and corn crops under the conditions of the studied area. According to this approach, irrigation interval was about 23 and 27 days for wheat crop under adequate and inadequate drainage conditions. The corresponding values for corn crop were about 16 and 17 days, respectively for the same two sites.
Also, the prediction of the WTD at the moisture content (MC) of the lower plastic limit (LPL), in the tillage soil depth (0-30 cm), could be used as an approach for soil workability using the simple measure of the WTD. According to this approach the suitable time for field operations is the time at which WTD reach 113 cm under wheat and 95 cm under corn, for the adequate drainage conditions. And at which WTD reach 72 and 63 cm, respectively, for wheat and corn under the inadequate drainage conditions.
The average workability timing duration, after an irrigation event varied between about 13 and 18 days, respectively, for corn and wheat in site (1) cultivated under adequate drainage. While it varied between about 15 and 20 days, respectively, for corn and wheat cultivated under inadequate drainage conditions in site (2).
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