Technical Evaluation of Subsurface Drip Irrigation System Hydraulic and Influence of Its Efficiency on Olive Growth Parameters under Field Conditions (Case Study: Isfahan)

Subsurface drip irrigation is one of the sub-surface irrigation methods which, despite its high costs, has become increasingly important today. The water distribution pattern is not visible in this system. However, a survey on water distribution model allows it to achieve the expected pattern with the design and operation management predicted. However, the mismatch of water distribution with a subsurface drip irrigation system may be due to many factors such as pressure changes, changes in emitter production, emission sensitivity to clogging, temperature effects and others cases are relevant, but hydraulic properties are the most important. Evaluation of a drip irrigation system under field conditions is important in order to ensure uniform distribution of droplets and to prevent them from clogging and uniformity of growth in the field.  It can also be effective in optimizing water use.

A study was conducted to evaluate the performance of the subsurface drip irrigation and its effect on the growth parameters of olive trees in Isfahan province during 2010-2013. Therefore a manifold system was randomly selected from the irrigation system and four emitter laterals were located along it; one near the inlet, two near the third points, three near the two- thirds points and the fourth near the outer end. Every year, the selected lateral pipes were uncovered and the water flow rates of all the emitters (108 emitters) were measured to calculate average emitter discharge (qavg), Christiansen uniformity coefficient (CU), emission uniformity (EU), manufacturer’s variation coefficient (CV). To calculate the volume of water consumed, potential evapotranspiration (ETo) was determined using daily meteorological data and by the FAO Penman-Monteith method. Then, using olive crop coefficient (Kc) at different growth seasons for Isfahan region, the amount of irrigation water was calculated based on plant water requirement. To measure the performance of garden trees, In addition to 16 trees mentioned above, 32 other trees were selected from the other two semi-main tubes and their yield was measured (48 trees in total). Soil sampling was carried out to study the moisture distribution in the irrigation system at three points, beginning, middle and end of a sub-tube, 36 hours after irrigation, and the samples were transferred to the laboratory to calculate soil moisture content.The results of emitter flow rate measurements indicated that approximately 6 to 10% of the emitters in every lateral were clogged. The reason for this was the rooting of the mulberry trees and rose shrubs, which were planted sporadically among the olive trees. Hence, the foregoing indices were calculated for two conditions; with and without the clogged emitters.

Considering the clogged emitters, the average values of three years of Qavg, CU, EU, and CV indices were 3.8 Lit/ h, 78, 72 and 16.4 % respectively, and in the case of clogged emitters were equal to 4 Lit/ h, 82.5, 75.5 and 15% respectively, and according to ASAE classification, the latter measurements (i.e. excluding the clogged emitters) were evaluated as “acceptable. The moisture distribution profiles indicate that the soil moisture has lasted to a depth of approximately 90 cm, and a higher accumulation of moisture was observed  at a lower depth due to the lighter soil texture in the upper layer (up to a depth of 60 cm) compared to the lower layer (8% clay versus 21% clay). Also changes in moisture to a distance of about 70 cm from the lateral in different depths is almost the same (the lines with the same moisture are parallel to each other), but from this point on, moisture decreases. The effect of moisture radius was observed to a distance of 60 to 70 cm of the emitter. The results related to growth parameters indicated that in contrast to the data of the first year, the average height, trunk diameter, canopy perimeter and area for the third year were 34, 57, 24.5, and 54% larger, respectively. In the present study, the percentage of increase in height and diameter of the tree in the seventh year of growth was 17% and 22%, respectively.

It is concluded that the olive trees have shown an increasing trend in vegetative growth under the subsurface drip irrigation system. The average efficiency of olive water consumption during the three years of the experiment was estimated at 0.4 kg/m3. Given that the trees were young this amount can increase under good management conditions in the coming years. The evaluation indicators showed that the observed defects in the design resulted from the three main factors of the design, implementation and incorrect use of the system.