Περιβαλλοντική και τεχνικο - οικονομική διερεύνηση της άρδευσης γεωργικών εκτάσεων με εφαρμογή νερού στράγγισης
Authors: Γκότσης Δημήτριος 
Published: September 2021

Increasing water demands worldwide pushed to find unconventional saving watersolutions, on the main consumer, which is agriculture. A known water-saving techniquefor irrigation purposes is the reuse of drainage water. This method has been used fordecades, mainly in arid or semi-arid areas, to meet seasonal shortages in irrigationwater, but also for environmental protection, since the volume of drainage watercontaining agrochemicals is so reduced and the final recipient for it (lake , river or sea),is finally relieved. In Greece, the method of drainage water reuse for irrigation, nor hasbeen investigated in depth, or has been used as a water-saving method, at least on anorganized basis, although the water consumption in agriculture reaches 80% of the totaland there is a serious deficit οf precious water resources in many greek regions.The present thesis therefore, aims to contribute through integrated and feasibleproposals, on a better understanding of a complex problem, both environmentally andfrom a technical and economic point of view. It is structured in seven chapters. In thefirst three (introduction, quality of drainage water and soil water salinity simulationmodels), one can find an extensive literature review regarding the subject, this thesisdeals with. The proposed algorithms for solving the problem, are presented in detail inChapter 4. Chapter 5 (experimental part), contains a global description of theapplication area, with references to its irrigation and drainage networks, the prevailingcrops, climate and soil conditions. Moreover, an elaborate presentation of theexperimental procedure is included. Chapter 6 presents in detail the design of theproposed drainage water reuse system for irrigation. Finally, the essence of the originalfindings of this thesis is contained in the general conclusions of Chapter 7. Two types of drainage water reuse practice are distinguished in literature: natural andartificial reuse. Natural reuse is caused when drainage networks are intercepted bynatural water streams and lakes, which supply irrigation water to downstream regions.Artificial reuse is man-made and can be divided further into two other forms, accordingto the extent of technical knowledge and management options of the selected system:Official and Unofficial reuse. Unofficial reuse is a self-sufficient practice that is held byindividual farmers, who use drainage water for supplementing their irrigation watersupply, without proper organizational background and expertise.On the other hand, in the official reuse systems, the appropriate infrastructure for therecycling of drainage water is carefully designed and implemented by the authorities, incombination with existing land reclamation works (irrigation-drainage networks). Themost common official reuse systems are the following:1. Reuse system through mixing drainage water with irrigation water of good qualityand low salinity (dilution of drainage water). It can be applied to existing irrigationdrainagenetworks by collecting drainage water in special reservoirs for bettermanagement.2. Reuse system without mixing drainage water with irrigation water. In this case,drainage water is used for irrigating crops with sequentially increasing tolerance tosalinity. The effluents of drainage water are minimized and then, finally disposed toevaporation ponds. A variation in this case, is to use alternatively pure irrigation waterand drainage water, depending on the crop growth stage. This system can be combinedonly with open channel irrigation networks.Either technique, apart from saving fresh irrigation water, can contribute to thedisposal problem of drainage water in areas where it is excessive, causing the rise ofthe water table and therefore, ventilation and salinity problems in the rootzone. Moreover, the use of drainage water for irrigation, results in the reduction of its volume.This minimizes the environmental risk of disposing a significant amount of agrochemicalsin the downstream natural recipient.The difficulty in implementing drainage water for irrigation is its degraded quality,particularly its high salinity, derived from the leaching of the salts in the soil root zone.The complex processes associated with the accumulation and transport of salts in the water-soil-crop system, is assessed through steady-state or transient-state, physicallybasedmodels. The latter are significantly more efficient for simulating the physicalprocesses, however a large amount of input data is required.What is to be found, should be focused on a practical methodology that will take intoaccount both the environmental, and the technical-economic aspects for theimplementation of the drainage water reuse method for irrigation, exploiting the existingexpertise and installed equipment (e.g. irrigation and drainage networks), seekingthereby the lowest possible cost of construction and operation of an effective drainagewater reuse system for the application area. The objective of this thesis is the creationof such a methodology, through the appropriate original theoretical tools, in combinationwith a sufficient base of experimental data, concerning both soil and water samples,collected in the study area and then, processed and analyzed in the laboratory.In particular, it is proposed:1). Determination of Crop Water Salinity Production (CWSP) equations from an agrohydrological,transient-state, physically-based model. Herein, these equations werederived with the use of simulation results from the internationally acclaimed,transient-state model, SWAP. The CWSP equations are the basis for the design andoptimization of drainage water reuse systems for irrigation, whether they use mixingor not, because they directly link the production of every crop, mainly, with qualityvariables of the application water.2). An algorithm 0-1, setting the irrigation cycles with or without the use of drainagewater with environmental criteria, related to restrictions, such as, for example, not exceed a maximum value of the salinity of the water applied, relative to the toleranceof the crop. For each crop are considered different importance weights in eachirrigation cycle, depending on its water needs and the growth of the root system,which, as has been demonstrated experimentally, helps to increase plant tolerance tosalinity. The weights are included in the algorithm’s objective function. Maximizingthis function, i.e. the reusable volumes of drainage water, based on constraints, asmentioned above, it is the core of the operation of the algorithm 0-1.3). An economic optimization algorithm of drainage water reuse system by mixingwith fresh water, which, including construction and operation costs of this system, with decision variables the total volumes of irrigation and drainage water that can beused, aims at maximizing the net present value (NPV), indicating whether theinvestment for such a system is profitable or not.The area of implementation of the above mentioned tools was the region ofAgoulinitsa, an artificially drained lagoon in north-west Peloponnese (western Greece).It is a coastal area, lying mostly below sea level, located in the south of the outlet ofAlpheus river. The irrigation network comprises three pumping stations in series: A1,A2, A3, which draw water from the main irrigation canal, coming from the diversion damof Alpheus river, in the location Floka. The irrigation network is divided into as manyzones as the pumping stations, covering 633 ha each, from a total of 1900 ha of the netirrigated area.The key criterion for the selection of this region, was the problem of disposal of largequantities of drainage water in the Gulf of Kyparissia, a site with natural beauty andgreat potential for tourism development (adjacent to the famous Lagoon Kaiaphas). Thequantities of water drained from the agricultural land, contaminate the coastalenvironment of the Gulf with agro-chemicals, such as nutrients and pesticides. Thecurrent discharge into the marine environment is not only environmentally detrimentalbut also economically, as drainage water is lifted mechanically by a pumping stationnearby the sea, located approximately in the middle of the area. Moreover, the selectionof the site was based on the potential savings of irrigation water, especially for the dryseason (July-August), where almost the entire supply of the river Alpheus, which feedsthe irrigation network, is diverted for irrigation purposes. The conjunctive use ofdrainage and fresh water could be an effective method of confronting with the disposalproblem and reducing also the risk of water deficit throughout the dry season. The first step, before applying the proposed methodology, was the creation of anenvironmental database for the chosen area. This was dictated from the lack ofprevious investigation and the need to produce up to date input data for the modelsused. These data include historical information of the region, data from previous studiesof the irrigation and drainage networks, as well as rainfall and meteorological data of thenearest stations of Pyrgos city from the Hellenic National Meteorological Service andthe National Observatory of Athens. Another memorable element of originality of this thesis is the production of asignificant number of experimental results, using samples of water from drainageditches and the main irrigation canal, as well as soil samples, from various points ofinterest in the area, both for the pre- and post-irrigation periods of the year 2010. Thesesamples were analysed with the existing equipment of the Land Reclamation Works andWater Resources Management Laboratory of the School of Rural and SurveyingEngineering of the N.T.U.A. In parallel, the area’s drainage network was digitized in GISenvironment and using the ArcGIS software, topographic and thematic maps wereproduced. These maps show the precise sampling spots identified in the field with GPS.Regarding the soils of the region from various parts of the whole irrigation zone ofAgoulinitsa, using special laboratory equipment (pressure plate extractors andcombined permeameter), their hydrodynamic properties (i.e. soil water retention curveand hydraulic conductivity), were determined for each soil sample. Because of thestrong spatial variability, average curves of the hydrodynamic characteristics wereobtained, corresponding to a representative soil type of the area, which is, according tovan Genuchten, a loamy soil.As far as the results from the experimental procedure and treatment of water and soilsamples, concerning their quality parameters (determined by the portable multiapparatusYSI Professional Plus and ion chromatography), the following was created:Thematic maps of drainage water salinity (TDS).Thematic maps of nitrate concentration in drainage water.Thematic maps of concentration of sodium in drainage water.Thematic maps of general suitability of drainage water for irrigation, covering thewhole 1900 ha of the region, according to the most acclaimed guidelines.Thematic maps of soil salinity and concentration of chloride and sulfate ions in the soil.All the above mentioned maps are "twins", namely, the water and soil status of theregion is shown, before and after the irrigation period (May-October). According to thesemaps, the following remarks can be made: 1. From the quality analysis of samples ofdrainage water and soil, it was shown that more burdened in terms of salinity andchlorides, are the north and the central zones (areas of A1 and A2 pumping stations, respectively). Something that is also obvious in maps with indications of suitability or notof drainage water for irrigation use. Furthermore, the types of salts detected in theregion, showed sea intrusion, especially near the coastline, where the main ditch. 2.The situation of drainage water from the point of view of salinity and alkalinity is better inpost-irrigation period. This can be explained from the leaching caused by the irrigationspreceeded. Nevertheless, nitrate concentrations are higher in post-irrigation period bothin water and soil, due to the use of fertilizers in the area. 3. Salinity in the surface layerof the soil appears increased in the post-irrigation period, which is due to the irrigationpractice in itself, even if the irrigation water used, proved that it meets the qualitycriteria. The situation improved in the pre- irrigation period, because of leaching fromwinter rainfalls. It is noteworthy that the average S.A.R. values in soil samples, werelow, but not in drainage water, because it has previously leached soil. iv). There areconsiderable quantities of agro-chemicals that flow annually into the Gulf of Kyparissia.About 28 tons of nitrate and 4 tons of phosphate, mainly due to the intensive use offertilizers in the region.Regarding the proposed drainage water reuse system in the region of Agoulinitsa,the key design criterion was to minimize the number and size of the necessary technicalworks, and therefore, the cost of these, combined with maximum use of the existinginfrastructure of the irrigation and drainage networks in the area. The following taskswere performed:1. Taking advantage of all available, relevant data (determined experimentally soilhydrodynamic properties and initial soil salinity, crops’ and meteorologicaldata), the CWSP equations for the prevailing crops in the region (cotton, maizeand alfalfa), were exported via SWAP model. The CWSP equations connectthe concentration of salts in the water applied for irrigation, expressed asspecific electrical conductivity ECAW (dS/m), with the relative yield RY of eachcrop. The chosen dependent variable was RY because it is dimensionless,more practical for the methodology proposed and more easily understood.2. From the algorithm 0-1 it was shown that, the application water mixture(average salinity of drainage water of 5 and 7 dS/m as scenario one andscenario two, respectively and average salinity of fresh water of 0.5 dS/m, at a mixing ratio of one to three), can be used in all irrigation cycles with minimumrelative yield of 100%, only for cotton. On the contrary, for maize and alfalfa,irrigation cycles with drainage water in the mixture, especially of 7 dS/m, arelimited to very few, for keeping a minimum relative yield of 100%. If theminimum relative yield threshold dropped to 90%, with drainage water of 5dS/m in the application water, the two crops could be irrigated in all cycles.However, with drainage water of 7 dS/m in the application water, except cotton,only alfalfa can be irrigated in all cycles for a 90% relative yield. On thecontrary, the maize, from the 9th cycle and after, demonstrating the beneficialeffect of the development of the root system in increasing tolerance to salinity,even for a very salt sensitive crop, such as maize.3. From the morphology and design of the local drainage network, it seems thatthe only area ensuring a low-cost and effective recycling system, is the one ofthe pumping station A3. Morever, from the water and soil samples qualityanalysis, it was apparent that this area was less burdened with agro-chemicalsthan those corresponding to the A1 and A2 pumping stations.4. The total water needs for cotton, per application and per irrigation zone, areabout 255000 m3. For environmental reasons and for the preservation of theexisting network equipment (e.g. sprinklers, pipes and motors), the mixing ratioof drainage water to application water, is not permitted to be greater than 1/3 ofthe total water needs of this crop, i.e. 85000 m3 . Therefore, the net capacity ofthe proposed storage reservoir for drainage water is proposed to be 17000 m3(five days duration of application x 17000 = 85000 m3 ). The proposed reservoir will be fed by the southern branch of the main drainage ditch, which drains the area of the pumping station A3. The filling time of the reservoir was estimated in less than 24 hours based (for safety reasons) on the minimum daily flow, estimated from data concerning the existing pumping station for dischargingdrainage water into the sea, which was estimated as equal as 20000 m3 per day, per irrigation zone. From this reservoir, the collected drainage water will be pumped through a D500 PVC pipe, of a total length of 1.27 km, upstream in the main irrigation canal. The power of the pumping station and the diameter of the feeding line, were chosen according to the daily supply of drainage waterrequired, during the 18 hours of the network’s daily operation (17000 m3 per day or 944 m3 /h). The drainage water will be significantly diluted in the main irrigation canal after being mixed with fresh water from Alpheus river. Flowing at a distance of almost 2.0 km, it will end up to the pumping station A3 and applied for sprinkler irrigation.5. One of the advantages of drainage water reuse, according to literature, is that, due to the increased load of agro-chemicals, less fertilisation to the agricultural land is required. Thus, on the whole, the agro-chemical load is expected to be decreased in the application area. Besides, from the nitrate concentration map produced from the treatment of drainage water samples (Chapter 5 -experimental part of this thesis), it appears that the nitrate load in the drainage water sampling points for the irrigated area of the pumping station A3, is up to 5 mg / L, i.e. the lowest ranking scale.6. In conclusion, the tangible environmental benefit from the proposed drainage water recycling system in the network of Agoulinitsa, is the saving of 1.36 ∙ 10m 3 of clear water of the Alpheus river, which will help to protect the ecosystem downstream of the diversion dam. Note that the river flow almost diminishes during the summer months and especially, during July and August, because it is used entirely for irrigation. The environmental benefit of disposing less agrochemicalsin the Gulf of Kyparissia, is equivalent, since 1.36 ∙ 10 6 m 3 less drainage water will be disposed (16 irrigation cycles x 85000 m3 ) .7. Incorporating construction and operation costs of the proposed drainage waterreuse system in the economic optimization algorithm, it was proved that this investment, combined with the cultivation of cotton, shows greater net present value (NRV) than the current situation (irrigation entirely with the water of the Alpheus river). Combined the investment of the proposed reuse system with the other crops (maize and alfalfa), turned out to be not attractive (NPV smallerthan that of the current situation). The price of irrigation water must be over three times the current, which is unlikely to happen in the near future, to be advantageous to use drainage water for maize and alfalfa with diminishing yields (95% for alfalfa and 90 % for maize). Therefore, the proposed drainage water reuse solution in the irrigation area of the pumping station A3, exclusivelyfor the cultivation of cotton, except the big environmental benefit for the area of application, as mentioned above, it is directly economically advantageous for producers and users of the irrigation network.