A Hydraulic Evapotranspiration Multisensor

Abstract

An exclusively mechanical stand-alone automatic device, self-adjusting to weather changes for controlled irrigation, that operates only on the energy of piped water, without electricity, is the described low-cost “Hydraulic Evapotranspiration Multisensor-HEM”. It is composed of an Evaporation Pan with water left to evaporate, a Floater with a Magnet floating in this water, a Hydraulic Device managing a Hydraulic Water Valve having means to adjust irrigation frequency, and a system that returns water to said Pan, through an Adjustable Dripper, to replace that lost by evaporation. During the Evaporation Phase, gradually the water level is lowered to a predetermined level, at which the floating Magnet acts on said Hydraulic Device to start irrigation. Water from the irrigation line is returned to the Evaporation Pan at the proper for the irrigation time rate. When the lost water is replaced irrigation is terminated and the system resets. On installation Irrigation Frequency and Irrigation Time are set with two graduated screws, for normal weather and the conditions of the particular plantation. HEM responding to weather changes modifies the irrigation schedule set, either by shortening, at a high evaporation rate, the time interval between consecutive irrigation cycles to protect plantations from water deficit stress or extending this time interval at a low evaporation rate to save water. Assessing the performance of HEM, by taking the estimations of evapotranspiration from the Penman–Monteith method shows high accuracy in the studied site. Considering the advantages of the product against the programmable irrigation controller devices, HEM provides optimum irrigation control in field crops and makes it a powerful “green tool” to be used in Mediterranean greenhouses.

1. Introduction

Maybe one of the most important factors for crop growth and sustainable water management is the determination of the right amount and frequency of an irrigation event (i.e., irrigation scheduling). Traditionally, irrigation scheduling was based on growers’ perspective rather than on specific microclimatic conditions, plant indicators (i.e., crop temperature), or soil properties, resulting many times in suboptimal growing conditions and a percentage of water and nutrient amount outflow to the environment [1]. Indeed, several methods have been used to calculate the optimum water volume of irrigation water, delivered at appropriate intervals to the various plantations, for the different growing stages for open field crops and within greenhouses [2,3]. However, in all methods evapotranspiration (ET_C) is the most important factor used for calculating optimum irrigation scheduling [4]. This is the combined water amount consumed through the transpiration (T) of the plants and the evaporation (E) from the wet soil. Models and methods of predicting soil–plant–atmosphere water transfers may be helpful for better management of water inputs to crops [5].

One of the most widely used systems for climatic measurements, in order to determine evaporation, is the Class A Evaporation Pan (i.e., 121 cm diameter) [6]. It has been used for estimating the amount of evapotranspiration in irrigation on a daily basis or even longer periods, for open field and greenhouse cultivations. The simplicity and high degree of adaptation on farm’s levels, which are characteristic of the Evaporation Pan, seem to be the primary reason for the pan expansion between producers; even though within greenhouses smaller pans or atmometers are preferable, as they occupy less space [6,7].

As ET_C depends on weather conditions (temperature, wind velocity, relative humidity, sunshine), as exactly happens with the evaporation rates of water from a free surface, the easiest method to calculate ET_C is to use readings obtained from an Evaporation Pan, relate them with a pilot plantation and apply certain factors for each particular plantation [8]. Based on this knowledge and considering the dependence of these two natural processes, namely transpiration in plants and water evaporation from a free surface, on the same weather constituents, the idea of controlling irrigation by monitoring evaporation, described in the “Hydraulic Evapotranspiration Multisensor, HEM”, is considered a promising irrigation system to produce. HEM operates as the controlling component of automatic irrigation systems, responding to weather changes by adjusting the time interval between consecutive irrigation cycles, set on installation for average weather, as the normal, for certain plantation and soil texture. The adjustments are automatically carried out by the Evapotranspiration Multisensor, for the purpose to save water at times of low evapotranspiration rate, but also to avoid water deficit stress in weather conditions imposing increased evapotranspiration rate. HEM senses the changes of the weather constituents (rainfall, temperature, humidity, wind velocity, sunshine) in a holistic way, thus more efficiently. The Hydraulic version is applicable where pressurized irrigation systems, i.e., drip or sprinkler irrigation are practiced no matter whether electric power is available or not. It is relevant to the work of [9] who proposed a prototype consisting of a mini-pan and probes, especially designed for greenhouse cropping conditions, for automatically governed irrigation in accordance with a water level. However, till today, despite that many sensors and technologies are available for monitoring the water level within a pan, they are still not compatible with the available irrigation automation [10].

“Hydraulic Evapotranspiration Multisensor” (HΕΜ) irrigation controller, which is an upgrade of the said “Automatic Irrigation Regulator Controlled by Water Evaporation”, is a novel product, because no similar Irrigation controller operating only from the energy obtained from piped water, that takes into account all weather parameters in a holistic way by monitoring water evaporation in an Evaporation Pan has yet been produced. Knowledge of automatic mechanical irrigation controllers originates from patents: “United States Patent, Kypris, Patent No 4,967,789, date of patent Nov. 6, 1990” with the title “Automatic Irrigation Regulator Controlled by Water Evaporation”, the “Numero de publication internationale WO 2006/058976 A1”. Date of Publication 08.06.2006, inventor: Ballet, Bernard, under the title “Stand Alone Mechanical Devices for Controlled Watering ” and the “Cyprus Patent No CY2620, Dedalos Kypris, 24 April 2019”, under the title Evapotranspiration Multisensor.

The general objectives of the production of a novel Hydraulic Evapotranspiration Multisensor, naturally adjusting to weather changes, is to provide optimum irrigation to plantations and also save water, to operate at remote places without electric power, but driven only from the pressure of piped water, controlled by the natural process of evaporation.

2. Materials and Methods

Product Design and Operation

Hydraulic Evapotranspiration Multisensor (HEM) is illustrated in Figure 1. It consists of a nonmagnetic Evaporation Pan (1), containing water, the pan bearing a nonmagnetic Base Pipe (2) tightly fixed at its base so that when the Pan is in position, the Pipe is vertical. A cylindrical Floater (3) with a tubular opening along its axis, housing a permanent Ring Magnet (4) floats in Evaporation Pan’s water, sliding on Base Pipe that enters said tubular opening. Within the Base Pipe, the tubular nonmagnetic Stem (5) of a Hydraulic Magnetic Valve is inserted. Below the Evaporation Pan is the nonmagnetic Body (6) of the Valve. This has two openings, the upper one as Inlet (7), and the lower one as Outlet (8). The two are connected through a small diameter Nozzle (9) closing by the weight of a Magnetic Rod (10), freely moving up and down in said Stem. The Inlet (7) connects through a small pipe (11) with the membrane Chamber (12) of a Transformed solenoid Water valve, that HEM controls (or to a Hydraulic Watervalve), conveying the hydraulic signals. When the Nozzle is closed the “pressure” signal is transmitted to the water valve to stop irrigation, when open the “no pressure” signal is transmitted to start irrigation. The Outlet connects to the irrigation line, with a similar pipe (13), either directly, or via the transformed water valve through the Adaptor (14), that replaced said solenoid. Said Rod consists of a nonmagnetic Stick (10) with a Rubber Pad (15) at the lower end, to close the Nozzle and a Magnetic Element (16) at its upper end. This Magnetic Element is attracted by the Ring Magnet (4) to open the Nozzle. The under-pressure water escapes through the Nozzle to the irrigation line, effecting a pressure drop in the Chamber (12), that opens the water valve. This happens when the water level in Evaporation Pan, due to evaporation, drops, down to a predetermined point, where the downward movement of the Floater with the Magnet is stopped by a Ring Screw (17) at the lower part of the Base Pipe. During irrigation water returns through a Return Pipe (18) and an Adjustable Dripper (19) to the Evaporation Pan, to replace what was lost by evaporation. The rate of Returned water defines irrigation time. The Returned water gradually raises the water level in the Evaporation Pan, and the Floater with the Magnet follows dragging upward the Rod until this is stopped by the Frequency Screw (20). This is placed, watertight, at the upper end of said Stem (5) to change, at will by turning, the route length that Rod is allowed to go, before being stopped by the Screw and falling down to close said Nozzle. The allowed route defines the irrigation frequency, which depends on the column of the water to be evaporated at each irrigation cycle. Said Frequency screw is graduated (21) in mm for setting the expected water level drop in the Pan on an average hot day as the normal, for the frequency-time between consecutive irrigation cycles we wish. The actual frequency depends on weather conditions.

The technical knowledge, after which the product HEM is based, is here explained. A magnetic hydraulic valve is actuated by a permanent magnet to open, when the magnet is brought near the magnetic component of the valve, while this is closed when the magnet is removed, with no electricity involved. An evaporation pan contains water, left to evaporate, while a floater bearing said permanent magnet floats in the water, following the dropping water level, at the rate imposed by the weather conditions. When the magnet approaches the magnetic component of the magnetic hydraulic valve, which is fixed at the bottom of the pan, turns it open. So water is released for irrigation while at the same time some water from the outlet of the magnetic hydraulic valve returns to the Evaporation Pan to replace what was lost by evaporation, raising the magnet away from the said valve to terminate irrigation. The rate of water evaporation determines the time between two consecutive irrigation cycles. Frequency is then automatically adjusted as weather conditions change. Irrigation time depends on the water return rate replacing what was lost by evaporation and is controlled by an adjustable dripper. HEM is suitable to control pressurized irrigation systems such as drip and sprinklers.

For the purpose of avoiding the preparation of complicated expensive molds, we proceed with simple, not costly, modifications, mainly on selected hydraulic valves of various types and sizes and Evaporation Pans. To test the suitability of the modified said existing components selected, a complete HEM system was prepared (Figure 2). The rest of the components were manufactured in a workshop from plastic or metal, to assemble several prototypes. Performance tests carried out at first in the laboratory, in relation to operational pressures, permanent magnet attraction force in relation to said Rod’s magnetic component, Return water methods and materials, and carry out pressure drop measurements. The fittings for adjustments, relating to irrigation frequencies and water return rate, relating to irrigation time, were tested and modified to be easily adjusted by the farmers.

The HEM device was then preliminary tested under high evaporation rates (summer period), in an open field side (Figure 3), in an inland semi-arid location of southern Cyprus (lat. 35°8′8.7 N, long. 33°24′9.6 E, altitude 165 m, m.s.l). Preliminary results will be used in order to re-adjusting the HEM components and increased the instrument sensitivity for monitoring lower within the greenhouse evapotranspiration rates. As regards water evaporation rates, for the average weather conditions in Cyprus (a semi-arid Mediterranean country), evaporation in summer time is about 270 mm per month (9 mm/d) and in winter about 70 mm per month (2.3 mm/d), while, for example, monthly water requirements of citrus plantations, or bananas, are for August 150 mm and 200 mm, for March 20 and 25 mm, respectively. Within greenhouses, the estimated evapotranspiration rates are estimated by 2–3 mm for cucumbers and tomatoes [1].

3. Results and Discussion

In irrigation technology all efforts have been devoted in the line of designing and producing electronic automated systems ranging from low-cost programmable timers to more expensive systems, monitoring ground moisture. Recently there is a trend in industries to move towards more sophisticated systems, which is to connect sensors, monitoring certain weather constituents (Figure 2). These are provided with a software to intervene and adjust the settings of the electronic timers, according to the readings of the sensors. Such systems are expensive, and difficult for an ordinary farmer to understand and manipulate. This HEM is easy to be installed and adjust by simple means, both the time interval between irrigation cycles and the duration of irrigation, which an ordinary farmer can do. The HEM product itself is an innovation because no stand-alone mechanical automatic irrigation system is yet available in the market. It has simple components, cooperating together in a way that is innovative. For instance, a floater bears a permanent magnet, which by following the dropping level of water in an Evaporation Pan acts on a special mechanism to start irrigation and following the rising water level in the same pan, because the water returns to replace what was lost by evaporation and acts on the same mechanism to stop irrigation. The mechanism is also an innovation, consisting of a vertical pipe within which a magnetic rod with a rubber pad is housed, free to move up and down when attracted by said magnet, having at the lower end fixtures to replace the solenoid of a common solenoid water valve, to perform as said valve without electricity.

Certain facts are worth considering for HEM to be preferred instead of other sophisticated irrigation systems or traditional time clock scheduling such as:

• When plantations have increased water requirements, because of weather conditions, irrigation frequency is increased to satisfy the demand, protecting plantations from water deficit stress.
• During cold or rainy days or other weather conditions, when water demand is reduced, the time between consecutive irrigation cycles is increased for water saving.
• HEM operates without the need for electricity of any kind, very much suited for remote plantations.
• The automatic operation based on natural processes, allows the system to operate without the need to be attended to for a long time, if there is no need to adjust the water volume required due to the plantation’s stage of development, or other reasons.
• In case the water supply is cut during irrigation, the system waits until water returns back and continues to complete the irrigation cycle, supplying also what water is due because of evaporation at the time the system was idle.
• The flow rate of the return water to the Evaporation Pan, to replace what was lost by evaporation, being transferred from that released for irrigation, is affected as the yield of the irrigation installations from pressure changes. In case of reduced pressure, irrigation time is extended to cover the loss.

Preliminary results proved that the HEM is quite robust to climatic changes; therefore, it could be used in precise irrigation control—matching the frequency of irrigation to the seasonal demands of the plants. Indeed, the mean daily potential evapotranspiration estimated considering climatic data of the area using the Penman–Monteith equation was about 6.5 mm for a representative week period in July (automatic agro-meteorological climatic station, MeteoSense 4.0, Netsens, Calenzano, Florence, Italy), which is very similar to the readings of the tested HEM (i.e., 7 mm d⁻¹). Deviations are expected according to weather deviations from normal. The following table (

Table 1. Water return rate in the HEM pan (mL min⁻¹).

Irrigation Frequency (Days)	Irrigation Duration (Minutes)
	15	30	60	90	120	150
1	21	10.5	5.3	3.5	2.6	2.1
1.5	31.5	15.8	7.9	5.3	3.9	3.2
2	42	21	10.5	7	5.3	4.2
2.5	52.5	26.3	13.1	8.8	6.6	5.3
3	63	31.5	15.8	10.5	7.9	6.3

Pan diameter 24 cm; floater diameter 8.8 cm; evaporation from pan set as the normal in Summer 7 mm d⁻¹.

) is for the dimensions of the tested HEM relates to the on-site adjustments, necessary for the system to deliver the correct volume of irrigation water.

The performance operation evaluation of the HEM, allowed us further proceeded with the model modification for testing the applicability of the HEM within the greenhouse. Research is in progress in order to increase the instrument sensitivity to greenhouse climatic conditions (smaller reference evapotranspiration rates) related to the Evaporation Pan diameter.

4. Conclusions

The Hydraulic Evapotranspiration Multisensor-HEM as described in this Technical Note was tested under field conditions and it was proved to be stable, accurate, and suitable as far as an irrigation water-saving device. With some adjustment needed in the field, considering the specific microclimatic, soil and related to crop growing conditions, HEM provides a smart stand-alone irrigation control system for crops grown under low-technology plastic Mediterranean greenhouses

5. Patents

“United States Patent, Kypris, Patent No 4,967,789, date of patent Nov. 6, 1990” with the title “Automatic Irrigation Regulator Controlled by Water Evaporation”, the “Numero de publication internationale WO 2006/058976 A1”. Date of Publication 08.06.2006, inventor: Ballet, Bernard, under the title “Stand Alone Mechanical Devices for Controlled Watering” and the “Cyprus Patent No CY2620, Dedalos Kypris, 24 April 2019”, under the title Evapotranspiration Multisensor (in Greek).