- freely available
Water 2016, 8(6), 234; https://doi.org/10.3390/w8060234
- Proposing a new methodology for determining the flows throughout the year in an irrigation network demand, considering the need of the crop, the historic consumption and the irrigation farmers’ habits
- Estimating the flow rate and pressures with the time
- Quantifying the energy balance in pressurized irrigation distribution systems to determine the energy footprint of water in the distribution system, and the estimated recoverable energy
- Applying these procedures to a real case study
2. Methods and Materials
2.1. Methodology for Determining the Flow
- Estimation of cumulative volume consumed by the irrigation point
- The determination of the irrigation probability (PI)
- i = numbers of days inside of interval;
- j = day of decision making;
- wdj = pattern to irrigate one particular day inside the interval;
- = total addition of patterns.
- The determination of the irrigation duration
- The start of irrigation
- Determination of irrigation volume
- Calculation of cumulative consumption
- The pressure and flow modeled for each node in the network
2.2. Balance of Energy
- = exchange of energy per unit time in the control system;
- = exchange of heat per unit of time (heat power);
- = power transmitted directly to or from the fluid (e.g., pump);
- = differential volume of control volume for integration;
- = velocity vector of fluid;
- = differential area of control surface for integration;
- = fluid density;
- = potential energy per unit mass;
- = internal energy per unit mass;
- = kinetic energy per unit mass;
- = height of pressure per unit mass;
- = time interval (s);
- n = total number of irrigation points;
- i = individual irrigation points;
- = specific weight of the fluid (N/m3);
- = total flow demanded by the network (m3/s);
- = piezometric head of the reservoir. For a pumped system, the value is the manometric height;
- = flow demanded by each irrigation point (m3/s);
- = piezometric head of the consumption node (m);
- = total energy (kW) supplied to the system. This term is equal to ET, which is later defined;
- = energy consumed by all irrigation points (kW). This term will be defined as ERI plus ETRI;
- = Exchange of internal energy. In an adiabatic system, it is equal to friction losses. This term will be defined as EFR.
- Total Energy (ETi): potential total energy in an irrigation point when the consumption is null in the entire network. It corresponds to the static energy (i.e., potential) of the node. For an irrigation point along a time interval, the value is:
- Friction Energy (EFRi): for a time interval, it is the energy dissipated in the network by the water coming from head until the irrigation point.
- Theoretical Energy Necessary (ETNi): it is the minimum energy required in a hydrant or line to ensure the minimum pressure of irrigation in the more unfavorable point. The value is:
- Energy Required for Irrigation (ERIi): during an interval of time, it is the minimum energy required at an irrigation point to ensure the irrigation water evenly. The value is:
- Theoretical Available Energy (ETAi): it is the available energy for recovery in a hydrant or line. The recovery coefficient in a hydrant or line (CRT) depends on losses existent between the hydrant (or pipeline) and the most disadvantageous consumption node. It is equal to the sum of the theoretical energy recoverable plus the theoretical energy unrecoverable (ENRT). The value of this energy for a particular time duration, is defined as:
- Theoretical Recoverable Energy (ETRi): it is the maximum theoretical recoverable energy in an irrigation point, hydrant or line of the network, ensuring at downstream the minimum pressure of irrigation.
- Theoretical unrecoverable Energy (ENTRi): it is the energy in a hydrant or line on the network that cannot be recovered. This energy is necessary to assume the losses from the line or hydrant to the more unfavorable irrigation point.
- Recovery coefficient in hydrant or line (CRTi): it is the quotient between ETRi and ETAi in an irrigation point, hydrant or line of the network. It represents the proportion of recovery energy over available energy.
- In plot of cultivation—in this case, the private user needs to reduce pressure down to 30 m w.c. to carry out drip irrigation. Generally, the user installs a pressure reducer to dissipate the excess energy. This element can be replaced by a pico-turbine to generate energy for self-consumption. This energy can be used in remote-control system, cleaning of filters, lighting and others similar consumptions.
- In the hydrant pipe—when the hydrant supplies to flat topography, reduction of pressure can be done. In an operating network, this reduction is carried out with a pressure reducing valve. This recovery could potentially be done if a suitable turbine could be installed.
- In pipe branch—in networks with large extension and irregular orography, some parts of the network can achieve higher pressure than necessary, forcing pressure to be reduced on a pipe branch. Currently, this reduction is possible by using a reducing valve installed on this branch. These valves can be replaced by turbines or pumps as turbines (PAT)  depending on the system characteristics to increase the energy efficiency of the network.
3. Case Study
- To make use of records of the water metered in the irrigation points. There are records since 2003 (year that network began to operate). In each plot, registers were taken quarterly corresponding to the months of March, June, September and December.
- To calculate the flow design (water requirements) for each of the considered plot, according to the crop and characteristics of the irrigation installation (distance between drippers and type). The number of sectors is established depending on the area of plots. This has allowed an allocation of irrigation according to the existing installation (Figure 1).
- To perform interviews of users and operating staff for estimating farmer habits. The type of irrigation management at the annual, monthly, weekly and daily levels has been analyzed in this questionnaire. Based on these interviews, different consumption patterns have been established. These patterns take in to account the irrigation habits of farmers: weekly trend, maximum days between irrigations and irrigation duration (Inputs 2, 3 and 4 in Figure 1).
3.2.1. Historical Consumption Data and Probability Function
- The annual consumption is estimated for each irrigation point, grouping them in terms of similar consumptions. This classification has provided the distribution presented in Figure 7.
- In order to determine the distribution of daily consumption, specific weights for crops of citrus and olive needs to be considered, according to the registered consumptions. The monthly pattern of irrigation needs has been set taking into account consumer groups (Figure 7). On the one hand, the irrigation points with consumptions lower than 3584 m3/ha have been assigned the patterns of consumption under the name “Crop of Olive” (Figure 8). On the other hand, the irrigation points with consumptions higher than 4480 m3/ha have been assigned the patterns of consumption under the name “Crop of Citrus” (Figure 8).
3.2.2. Pattern of Irrigation Habits
- Analyzing the information obtained from interviews, two trends of irrigations have been depicted. Small farmers avoid Sunday as irrigation day and Saturday appears with double preference than the rest of the days (Figure 9a). Big farmers also have double preference for Saturday, but do not avoid irrigation on Sunday (Figure 9b).
- Distribution of maximum days between irrigations: these patterns refer to the maximum interval between watering. Irrigation occurs every day during the months of higher consumption (May, June, July, August and September). In remaining months, the intervals of irrigation increase, being not a clear and well-defined pattern for all farmers. Each farmer chooses the interval according to different factors (e.g., rain, availability and soil properties). Based on the results of the interviews, four distributions have been defined. According to the results of the requested data for farmer habits across surveys, patterns have been assigned. Pattern I has been assigned approximately to 40% of the irrigation points, pattern II to 20%, pattern III to 20% and pattern IV to the other 20%. This assignment has been carried out randomly (Figure 10).
- Patterns of irrigation duration: based on the requested information four distributions have been proposed. Again, pattern I has been assigned approximately to 40% of the irrigation points, pattern II to 20%, pattern III to 20% and pattern IV to other 20%. This assignment has been carried out randomly (Figure 11).
- Distribution of irrigation start probability: farmers tend to irrigate in certain particular hours of the day. This aspect is considered in this methodology by using the patterns for the probability of starting irrigation in the different schedules. The watering schedule between 10 A.M. and 4 P.M. is chosen in the months of January, February, March, April, October, November and December. However, farmers irrigate in different light hours in summer months to avoid warmer hours and night. Therefore, three patterns have been developed to define the probability (see Figure 1, step 4).
4.1. Basic Characteristics
4.2. Flows in the Network
4.3. Water-Energy Nexus Estimation
4.4. Theoretical Recoverable Energy
4.5. Global Energy Balance
4.6. Economic Feasibility
Conflicts of Interest
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