Journal of Irrigation and Drainage Engineering

Alfalfa provides high-energy feed for dairy cows and other livestock. This crop is also known for its high water-use efficiency. Subsurface drip irrigation (SDI) is an ideal method for meeting crops’ water demands. A major design issue in SDI systems is determining the appropriate dripline depth of placement to provide adequate irrigation. The presented approach used HYDRUS $2\mathrm{D}/3\mathrm{D}$ to simulate spatial and temporal distributions of soil moisture content, root water uptake, and drainage of an SDI system commonly used for alfalfa production in Holtville, California, in late summer. The scheme presented supports the design and management of SDI systems for alfalfa by preventing a wet soil surface at harvesting time, maintaining trafficability, delivering the required crop water, and minimizing water loss at the same time. Based on the simulation results and after a sensitivity analysis, a dripline depth of 50 cm is recommended to ensure a drier soil surface at harvesting times, with a run time of 12 h every three days, which satisfies daily crop water needs and minimizes water loss.

There are two types of flow in underground pipes: gravity-driven flow, such as in sewer pipes, and pressure flow, such as in syphon pipes. Defective pipes can cause extensive economic losses and have received considerable research attention; for instance, infiltration into sewer pipes through defects has been thoroughly studied. This study investigated the effect of a defect in syphon pipes on channel bed morphology; 162 experiments were performed for different Froude numbers, channel water depths, upstream heads at the defect, syphon discharges, defect discharges, defect diameters, and bed material depths above the syphon. Because syphon pipes are constructed beneath a channel bed, the characteristics and locations of the defects in syphon pipes are very difficult and costly to determine in practical applications. However, changes in channel bed morphology due to a defect in a syphon pipe can be measured. The results of this study relate the characteristics of a defect in a syphon pipe with the changes in channel bed morphology above the syphon. The results indicate that the defect diameter and Froude number are the predominant parameters that influence the channel bed morphology.

Alfalfa hourly bulk surface resistance ($r_s$) has been modeled as a function of crop biophysical characteristics. The new modeled $r_s$ allows the calculation of hourly alfalfa evapotranspiration (ET) using the Penman-Monteith (P-M) equation. In addition, another equation from the literature was also evaluated. The experiment took place during the 2009 and 2010 alfalfa growth seasons near Rocky Ford, Colorado. The results indicated that ET obtained from $r_s$ parameterized with leaf area index (LAI) or crop height ($h_c$) was more accurate than ET calculated with conventionally obtained $r_s$ when evaluated with lysimetric data. In addition, for $h_c$ less than 25 cm, ET was remarkably better estimated with the new $r_s$ model than with conventional $r_s$. However, for $h_c$ greater than 25 cm, the proposed $r_s$ was slightly better than conventional $r_s$. The value of conventional $r_s$ was underestimated when $h_c$ was less than 25 cm. These underestimated $r_s$ values resulted in higher hourly alfalfa ET rates (20% in 2009 and 41.5% in 2010). By contrast, the new approach underestimated ET by 7.5% for $h_c$ less than 25 cm in 2009, and ET was overestimated by 7%–9% for the 2010 data set. The accuracy of ET with conventional $r_s$ in 2010 for the same short $h_c$ was poor, because the Nash-Sutcliffe coefficient of efficiency was negative. Therefore, using a variable $r_s$ for alfalfa as a function of LAI or $h_c$ improved the estimation of alfalfa hourly actual ET.

Sharp-crested slit weirs are used for measuring small flow discharges. Different stage-discharge relationships can be found in the literature for computing the flow discharge of sharp-crested slit weirs. In this study, the performances of different available stage-discharge relationships are discussed using experimental data available in the literature. The flow process of a sharp-crested slit weir was analyzed based on the dimensional analysis theorem. The deduced stage-discharge equation was calibrated using the available experimental data for slit weirs with contraction ratios ranging from 0.017 to 0.25. The deduced stage-discharge relationship allowed measuring discharge values with an average error of 1.02% and a maximum error of 4.6%. The results of this study indicate that while weir Reynolds number is a predominant quantity affecting the flow discharge of rectangular slit weirs, the contraction ratio is a predominant quantity for contracted weirs. At the cost of less accuracy, an overall solution is presented in terms of the contraction ratio, which is applicable for slit, contracted, and suppressed weirs.