AbstractOptimizing rates of water application to irrigated crops is an important method for conserving water in arid areas. Trickle irrigation is one method of applying water to crops with minimal losses. This field study was conducted to determine chile (Capsicum annuum L.) growth and development when irrigated by trickle irrigation at different levels of water application. Chile peppers were grown on a 2‐ha field irrigated with a trickle irrigation system. Water application rates of 0.8, 1.2, and 1.4 times a control treatment were maintained from mid‐June (74 days after emergence) to maturation in September. Throughout the growing season measurements were made of leaf area index, and of the accumulation of dry matter in stems, leaves, roots, and pods. The data show a clear response of the rate of leaf area development and dry matter production to water application rates. From the start of treatments the dryer treatments had lower rates of leaf area development and dry matter production, resulting in lower final yields of above and below ground plant parts. However, water use efficiencies varied little and were between 8.1 and 8.2 cm actual ET per ton of dry mass production. The results indicate that limiting the water applied to chile during the period of rapid vegetative growth reduces final yield, but has limited effect on water use efficiency.

AbstractThe present field studies were conducted to examine the response of chile pepper (Capsicum annuum L., var. New Mexico No. 6‐4) during one irrigation cycle, to different deficit trickle irrigation treatments.After applying four different water rates for 46 days, detailed measurements were made, over a 3‐day period between two irrigations, of the water content, the soil‐water pressure potential, the osmotic potential of the soil solution, the leaf water potential, and the stomatal resistance. Crop evapotranspiration was estimated from water balance measurements, and hourly potential evaporation, ET0, was calculated using meteorological information.There were four water treatments, consisting of a control treatment and three treatments receiving 80, 120 and 140% of the water applied to the control. There were considerable differences in water transpired from the different treatments, but only slight differences in plant water potentials and stomatal resistances. At any time the differences in plant water potential and stomatal resistances were less than 2 bars and 1 sec/cm, respectively. Chile appeared to adapt to the soil water supply by controlling its size. The plants in the dryer irrigation treatments displayed less leaf area, fewer leaves per plant and less aboveground dry mass production. Because of the large differences in plant size associated with relatively small differences in measurements of the plant water status of chile, it appears inadvisable to depend upon pressure chamber, or diffusion porometer measurements for irrigation scheduling of trickle irrigated chile.

Computer models are often used to simulate solute transport through soil profiles. This paper compares the results obtained with two such models. The first model is rather complex and requires a complete description of the soil hydrological properties. The second model is much simpler and uses an average percolation rate as input. Solute transport was compared with and without adsorption and with and without root water uptake. The results show that the relative solute concentration at a given soil depth, as a function of cumulative drainage of an intermittently irrigated soil, is a smooth curve, which can also be simulated with the simple model. Such models are apparently appropriate for predicting solute transport through field soils with their inherent spatial variability in physical and chemical properties.

For a loes soil with sugar beets ( Beta vulgaris L.) a model was developed with which the difficult-to-measure components of the water balance equation can be determined. Basic component of the model is the unsaturated soil moisture flow equation. This equation is solved numerically by a finite difference method. As boundary condition at the soil surface, expressions for the potential and the actual transpiration and for the potential and the actual evaporation are used. Besides the hydraulic functions of the soil, standard meteorological measurements, together with leaf area index and root distribution data of the crop, enter the model. For a period of 316 days, calculated and measured soil suction values in 10 different soil depths, are compared as well as transpiration and seepage vaules. The comparison shows that the model provides acceptable results. The model calculations indicate that of the 519 mm of precipitation received during the 316-day period, 503 mm were used for evapotranspiration and 119 mm left the soil profile as internal seepage.

AbstractA 218‐day experiment on a fallow loess soil was carried out to study infiltration and redistribution under natural field conditions. On an experimental plot of 6 by 6 m tensiometers had been installed in 11 depths: 10, 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 cm. They were read at short irregular intervals. The precipitation was measured daily. The hydraulic functions of the layered, horizontal soil and a relation between potential evaporation, actual evaporation, and the soil suction at 5‐cm depth had been established previously. With use of these soil and atmospheric data the unsaturated soil moisture flow equation was solved numerically. Purpose of the numerical analysis was to determine if the unsaturated soil moisture flow equation does describe observed soil moisture behavior for such a long period. It was found that the calculated soil suction values for all depths deviated less than 15% from the measured ones. It is concluded that the unsaturated moisture flow equation can be useful means to supplement field measurements on infiltration, redistribution, evaporation, and internal drainage.

A numerical method is developed with which the flow of moisture towards suction units in soil can be evaluated. For a number of selected problems the unsaturated moisture flow equation is solved twice: (i) for radial flow towards a suction plate or suction cup; (ii) for vertical downwards flow under the same conditions, but without a suction unit in the soil. The calculations are performed for a flat soil without a plant cover. It is found that even for little vacuum, large differences may exist between the seepage rate as determined from a suction unit as compared to the seepage rate of the unextracted soil. It is also shown that the radius of influence of a suction unit, which extracts water continuously, can be several feet wide. It is concluded that in order to be reliable, data collected with suction units should be analyzed with care.

AbstractThe moisture dynamics of an undisturbed soil monolith were studied during a lysimeter experiment. Daily measurements were made of the soil suction at 10 depths. Also daily measured were the precipitation, the seepage, and the evaporation from the monolith during a 3‐year period. For selected periods, a drying (desorption) curve and a wetting (sorption) curve of the soil moisture characteristic were determined from field data. Also the capillary conductivity was determined with use of daily monolith observations. With use of these hydraulic functions, the unsaturated soil moisture flow equation was solved numerically for one‐dimensional vertical flow. In order to determine the effect of hysteresis on the suction distribution in the monolith, calculations were performed either with the desorption curve or with the sorption curve without scanning between these curves. Neither of the two curves leads to complete agreement between observed and calculated soil suction values; the desorption curve usually gives too high values, the sorption curve too low values.

A device for the irrigation of soil columnsAn irrigation device was designed for maintaining a constant water flow in soil columns over a long period of time. The apparatus consists of three construction elements: A balance for the dosage of the water, a sprinkler to distribute the water on the soil surface and an electronic time control to adjust different time steps. The wide range of possible rain intensities is shown. The mean variation of the water pulses and their distribution over the surface are discussed. The apparatus exhibits good results in long duration experiments.

Soil hydrological methods for investigations on undisturbed samples of skeleton‐rich soilsNew methods are proposed, which allow the determination of moisture transmission properties of stony soils. A soil sampling technique is discussed whereby small monoliths are isolated from the surrounding soil and covered with polyester‐soaked glassfiber sheets. With use of such irregularly shaped columns the soil moisture characteristic and the unsaturated hydraulic conductivity are determined by laboratory procedures. Starting with a saturated column, suction is applied and the resulting outflow is measured. At the same time the suction in the sample is registered and also the hydraulic gradient within the sample is determined.Also discussed is a procedure to install tensiometers in stony soils. The installation provides reliable data and the maintenance is easy. By building in a heating system in the tensiometers, these can be operated all year round.