Needs for Research

Next, some topics for the management of water in the Chilean agriculture that, in the authors's opinion, require further investigation in the near future, will be proposed. In general case, these topics are consistent with some of the guidelines defined by the "Agenda for Agricultural Innovation", that were mentioned earlier. These examples mainly focus on the Limari Basin (Coquimbo Region, 30°30' S) and the Valley of Peumo (O'higgins Region, 34° S), which are representative of the conditions where agricultural irrigation activity is developed in the Chilean Central North and Central South areas, and where part of the research work of the authors has been carried out. However, it is important to clearly state that the analyzed situations are equally relevant, applicable, and valid for other areas of agricultural importance in Chile.

Soil-Plant-Water Interaction

Soil interacts with water and therefore its characteristics are a reflection of, and sometimes the reason for agronomical decisions. However, in some areas of the country the information on soil and the effects of the irrigation techniques over them is scarce. An interesting case corresponds to the "new" irrigation area in the Limarí Basin, Coquimbo Region, where an important increase of the irrigation surface over land traditionally considered "marginal", that is, areas above channel location and with important slopes, that sometimes rise over 30%, has occurred in the last years (Figure 4).

Along with the explosive and sustained agricultural development that have taken place in the last decades, it is important to mention that most of the existing soil surveys in the region are rather old (60's or 70's), and that they are concentrated on the at-the-time irrigated area. Although they are complete in terms of morphological descriptions, they are at the same time incomplete in terms of physical and chemical analysis. For instance, the La Paloma soil survey, for the Limarí Province, was carried out in 1967 (Alvarez, 2005; SAG-DICORA 1967). Also, the referred study was developed with the objective of the assessment of the soil limiting factors, in terms of their morphology, regarding surface irrigation methods typically used by that time such as furrow and flood irrigation. Thus, limitations were determined as a function of runoff and erosion, issues that should not be as important as other hydrological processes such as infiltration and redistribution when technified irrigation methods such as drip and sprinkle irrigation are used. Even more, the mentioned soil study covered areas that are currently under water (because of the construction of reservoirs) and also did not consider areas that are currently cultivated. In that sense, although the La Paloma system reservoirs (La Paloma, Recoleta, Cogotí) has been functioning for over 30 years, there is no updated database with records to guide the management of the irrigation. This situation, in association with the overall lack of detailed physical and chemical analysis, highly difficult the decision making progress oriented to a sustainable irrigation agriculture.

For instance, as described by Oyarzun and Alvarez (2001) and Alvarez (2005), it is rather common to find in these historically non-irrigated areas soils with fine textures and at the same time relatively very well structures of prismatic type. Thus, a hybrid hydrological behavior is faced: at low water contents (high tension), water circulates through the aggregates matrix, whereas at high water contents (lower tension), water tends to circulate through the between-aggregates spaces. This kind of behavior will affect also root aeration and distribution. Given the fact that roots are the places where growth promoter hormones such as Cytokinines are produced (Kramer, 1983), this should be finally related with above-ground growth (shoots and fruits).

Figure 4: Satellite images (false color) showing the evolution of agricultural land patterns (irrigated areas, in red) in sloped terrains of the Huatulame sub-basin, Coquimbo Region.

As a consequence of the discussed aspects, several questions arise, such as: What happens with the irrigation water movement in the soil?; What is the shape and distribution of the wet bulb and nutrients?; Which are the water accumulation zones?; Are irrigation design and management schemes considering these situations or just practices for flat terrains are followed?; What is the spatial and temporal variability of soil moisture?; When man-made soil ridges are developed, what are the physical properties of this mixed soil where trees are growing? Moreover, this subject becomes more complex when considering clayed soils, in particular regarding sensible species, such as avocado, in terms of water excess and soil oxygen levels.

The effects of situations such as those mentioned should be taken into account when irrigation systems and orchards are designed and planned in these new irrigated areas in sloped terrains. However, it is common to find irrigation systems that have been designed and installed following "standardized guidelines" developed in other regions and with other conditions, without any consideration of the local characteristics. Moreover, the effects of both cropping and irrigation practices in these highly intensive systems over the soil properties and behavior have not been extensively studied. Preliminary characterization on the sector of the Huatulame sub-basin, Limari catchment (Alvarez, 2005) allow us to mention several effects on the soil such as the beheading of the surface soil horizon, the modification of the stoniness distribution, and the modification of the slope configuration as three important changes induced by irrigated agriculture practices (Figure 5).

Figure 5: Soil surface and profile traits in the Huatulame irrigated area, Limari Province.

Likewise, it has been observed that the excessive labor over the inner rows determines the presence of a secondary compact horizon with scarce roots. In this case, the humidity -machinery use combination generates a flowerpot effect that limits the root useful volume, from the water and nutritional perspective. Another morphological trait of the soils cultivated with vines is the displacement of the original A horizon towards a central ridge. This has caused a different radicular distribution between the plant row (ridge of furrow) and the space between rows. In this case the "A" horizon from the inner row is moved from its original place towards the upper row generating an "Anthropic" profile with an "Ap" horizon with a higher exponent than the original.

Also, it must be considered that irrigation practices, i.e. water application, affect clay swelling/shrinkage processes and the temporal pattern of these oscillations in the soils. Along with this, another aspect that has been observed has to do with the effect of irrigation over the natural saline balance of the soil, adding dissolved salts to the water in addition to that of the fertilizers. Thus, with a larger contribution of fertilizers the salinity should increase in the root area, except that the washing rate applied with irrigation keep the original saline balance.

Evidence of accumulation of salts at the level of the third horizon has been observed, for some of the analyzed soils, probably due to the surface flow and the slow percolation occurring associated to the amount and type of clay, in particular smectites. The salt-leaching washing rate can, undoubtedly, be managed with irrigation, but it's limited by mineralogy and content of clays. The salt-leaching practice without control could generate anoxic problems (saturation) in addition to an increment in the water use. Finally, traces of eluviation (leached), illuviation (deposit) and the existence of primary and secondary minerals with a particular spatial arrangement have been found through micromorphologic analyses. The eluviation traces indicate that there is an important water dynamic in the soil, and that the inner flows mobilize clays from the superficial horizons towards the lower horizons. Eluviation is itself an evidence of fertility loss because of water (Alvarez, 2005). In summary, there are several components that have not studied yet and that we envision as strategic developing lines for the irrigated agriculture under similar conditions. Considering all these elements, it is possible to state that a proper understanding of these processes, at a local level, is currently required for a successful and sustainable irrigation management and the planning and application of related agricultural practices such as fertilization or soil salt leaching.

Part of this aspects, having a better understanding of soil hydrological processes and their implications in irrigation systems design and management, have begun to be taken into consideration in recently started projects such as the Project SIAR-Limarí (http://www.siar.cl/). The SIAR project is a three years initiative financed by CORFO (Corporación de Fomento de la Producción) through its Innova Program, and its general objective is "to technically assist producers in the programmed and controlled application of the resource, in order to adjust in real time, the dose and frequency of irrigation for the real water requirements of the different species of vegetables during their development". The aforementioned project considers different aspects of the agronomic system, being one of them the characteristics of soil (hydrologic behavior, depth, texture, density, salinity and physical chemistry composition in general). Even though this is the first year of the SIAR project, it has already been able to generate interesting soil information that will make possible to regulate the water applications made by agriculturists of the basin.

Change of Irrigation Practices an Increase of Irrigated Surface

Along with the soil-plant-water relationships at the orchard level discussed above, the current national trends in the increase of the irrigated agricultural surface, mainly on sloped terrains, with high efficiency systems (drip or sprinklers) have an uncertain hydrological effect at the watershed level (Figure 6). Indeed, from the hydrological point of view, previously in a watershed we have an agriculture area (let say X) that use an amount of water (let say Y) with an irrigation efficient of approximately 30%; that means that an important volume of water return to the watershed (about 70% of Y). In the future scenery we will have in the same watershed an agriculture area of 2X, with an irrigation efficiency of 90%, so the return of water to the watershed will be only a 10% of Y. That will means less water recovery of the rivers.

Figure 6: Newly established avocados on hillslope at the Aconcagua Valley (33° S) (Panel A) and the Elqui Valley (30° S) (Panel B)

Now that analysis takes us to the following research questions: What will be the effect of the change on agricultural and irrigation practices on the hydrological regime of the watersheds?; What will be the effect on water rights of the changes on water recuperation of the rivers?

To fully answer the previous questions there are some key issues that must be addressed: a) To develop of monitoring techniques that may work under Chilean reality. That means to considerer in the design aspects such as: i) relative low cost to make affordable the monitoring program; ii) reliability (and accuracy) of the data to be obtained, and iii) Safety and robustness in operation (simple to use and easy to camouflage to avoid vandalism); b) To develop hydrological models which can operate with sparse data. That development should focus on a robust conceptualization and integrated knowledge where scale will be an important issue.

Operation of the Water Market

Along with the expansion of the irrigation surface, another factor to be considered regarding the agricultural development under irrigation in Chile, refers to the declaration of over-allocation of the basins. Due to these situations, which prevents the constitution of new permanent exploitation rights of surface water, it is reasonable to think that a steady increase of the planted surface will require more water resources, which should come from the market (need for transparency, that will be discussed later on) or from the exploitation of new sources such as groundwater (need for regulation and control). Actually, access to the rights can be gained generally through four ways: inheritance, rental, buying, and constitution and inscription of a new right. For example, in the case of the Limarí Basin there is a declaration of over-allocation since February 2005, currently in force and that the DGA uses as a base not to authorize the constitution of new superficial exploitation water rights. Therefore, to gain access there is only the ways of the market or the exploration and application for the constitution of groundwater rights.

Related to the first, market and rights transference (need for transparency), it is worth mentioning that organizations such as the Junta de Vigilancia del Rio Grande-Limarí y sus

Afluentes and the Asociación de Canalistas del embalse Recoleta (both of the Limari Province) have regulated through meetings and directories the transference of rights intra and inter organization. Beyond details, what's important is the structural vision of the system so as to minimize the potential negative effects (external issues) derived from the mobility of the rights. What has been described has evident territorial implications, since the most rigid areas on their water demand sustain their development on the internal agreements of the La Paloma System, aspect that requires maximum transparency. Beyond the economic settlement and public-private negotiations, the System is expected to be fully operated, in the short-term, by privates represented by nine irrigation organizations. Under this context, and considering the responsibility that the private actors are willing to assume, it is possible to wonder how they will develop the controls tools that give feasibility and sustainability to the System, considering that these aspects are the basis of the integration of the basin through the water management and the rights related to it. In the same sense, the operation of the local irrigation organizations has incorporated the volumetric management of the rights but lacks of qualitative monitoring of it, as well as they lack of effective controls of transference and mobility of the rights, state of the installations, and the assessment of the whole efficiency of the System. So, how could the operational management of the basin and of the organizations be preserved and guaranteed, and that can be proven transparent and sustainable?

In particular, transparency on the operation of the basin must be transversal to the organizations allowing transferring to the bases, that is "the irrigation organizations", and finally farmer with water rights, the state of the management. For this reason, the generation of a base protocol for operation of the organizations and of the system as a whole is required. Besides, competent professionals must be trained to develop this function and, finally, constant information of the status of the management at the level of the water communities must be delivered and made public. In this way, the farmers with water use rights will themselves control quantitative and qualitative the integrated management of the basin.

For the above mentioned reasons, it seems important to propose the development of a specialized process of management tools, such as water audits, that ensure transparency and generates an integrated control of the water resources management in basins such as the Limarí. An initiative of this kind was recently submitted (January 2007) as a project to CORFO under the name of "System of Water Audits". The importance of the concept behind this idea is that on the regulatory and politic system that rules the water management in Chile, it is required the creation of supervision and control systems over the operation of the dams and installations of hydrological systems, especially when these will be managed by privates.

As to the second aspect, the use of groundwater, it is worth mentioning that several basins in the Central North area of Chile, area characterized by narrow valleys E-W oriented, with aquifers on porous materials of small dimension in the headwaters of the basin and of middle-size area in the middle and low parts of it, restricted to the recent alluvial deposits. In this restrictive hydrogeological context the legal and technical aspects discussed on the previous sections become relevant. Thus, the interaction of surface water - groundwater subject emerges as a relevant aspect. Despite the current legal separation, the interaction between surface water and groundwater is usually very active. It is particularly important to understand the participation of the surface and groundwater on the supply of the water needs of the basin. Systematic studies of the relations of interaction river-aquifers are essential to provide the DGA and privates with the necessary tools that allow a correct volumetric management of the water in the basins. One option involves the use of analytic expressions like the Jenkins method for the estimation of the river-aquifer interference, and for example it has been used by the organizations of users of the Limari basin to oppose the constitution of underground exploitation rights. Indeed, in the last 6 years irrigation organizations have opposed the constitution of underground exploitation rights for a total of 285 l/s, in 7 localities of the Limari basin. Another option is to use numeric models and/or stable isotopes analysis. However, the use of any tool depends on the availability of field information with respect to the hydrogeological characteristics of the system, which remains quite limited yet and in a rather coarse scale in several zones of the country.

Lixiviation of Agrochemicals in Orchards

Modern agriculture depends largely on agrochemicals, especially fertilizers and pesticides, in increasing amounts. Because of this, agrochemicals that are not assimilated by crops and those which are not degraded within the vadose area, become a source for potential pollution in groundwater, even when the improvement on the irrigation efficiency allows a decrease in the deep percolation. However, as Lovejoy et al. (1999) point out, the timing and spatial distribution of the application, the crop, soil, slope, hydrogeology, irrigation method, weather and management practice patterns are as important as or more important than the amount of fertilizer applied. Also, Troiano et al. (1993) carried on a study of tracers where it was shown that the distribution of atrazine in the soil profile depends on the amount of percolated water, establishing that gravitational irrigation, used in the valley in study, is the method that percolates the largest amount of water, while localized irrigation methods, like microsprinklers, lixiviate the smallest amount of pollutes. However, the prior does not imply that the election of an irrigation method can decrease in itself the risk of pollution, since the percolated water volumes depend on the operation and management of the irrigation systems.

On studies carried on the Valle de Peumo (O'Higgins Region) in orchards with pressurized irrigation (droppers, microjet), it was confirmed that irrigation generates a relatively constant humid area during the season, where fertilizers accumulate (Rivera et al., 2005 and 2007; Arumi et al., 2006). The areas of temporary storage can be explained by the non saturated mobility of water on soil and unevenness in the humidity-conductivity relation. During summer, under irrigation conditions, because of humidity differences and the bulb stability of the humid area, hydraulic conductivity K1 inside the bulb is higher than the hydraulic conductivity K2 in the area surrounding the humid bulb, generating an area of accumulation (Figure 7). During winter, pollutes on these accumulation areas could be transported nearby or under the phreatic level by the effect of rain pulses.

The previously exposed issues allow us to explicit some questions for work: how do the practices of fertirrigation affect the overload of contaminants to groundwater?; how should fertilizers be applied if you take into consideration aspects such as weather and phenological states of the crops or orchards?

Figure 7: Process of accumulation and transport of pollutes in the humid bulb under localized irrigation and precipitation conditions.

The modeling of the water and fertilizers balance in orchardds must be improved. Currently there are models like Cropsyst or Hydrus that allow for a study of the subject, but these models must be perfected, especially regarding the simulation of the dynamic of water and nutrient extraction from the roots by orchards.

Effect of the Distribution Network of Irrigation Water

Studies of the hydrological effects of seepage from irrigation canals have shown that seepage can be an important source of recharge to shallow groundwater in specific situations. Groundwater recharge from irrigation canals was shown to cause groundwater mounds directly beneath canals (Maurer, 2002), though larger valley aquifer effects were not documented. At the farm, canal and field seepage were shown to increase water tables during the irrigation season in northern New Mexico (Fernald et al., 2007). Larger valley effects have only been documented in few situations. In the North Platte River Valley, for example, stable isotope studies confirmed the irrigation canal seepage origin of recharge that caused a rise in local groundwater levels (Harvey and Sibray, 2001). Lining irrigation canals was shown to reduce the availability of shallow groundwater that supplied wells to irrigate cropland (Calleros, 1991). Modeling studies have illustrated the effect of canal seepage on shallow alluvial aquifers (Youngs, 1977; Yussuff et al., 1994; Ram et al., 1994). If lining canals reduces seepage rates, there may be less recharge to shallow groundwater. In terms of the scientific and technical literature, there is a need to fill gaps in the understanding of the effects of canal seepage on shallow groundwater and larger spatial scales (like the river valley) over longer temporal scales (like the full irrigation season). There is an acute need for improved hydrologic understanding particular to producers in Chile contemplating changed land and water use management.

In the Peumo Valley, irrigation seepage and groundwater recharge patterns were consistent throughout the valley and are illustrated with examples from an upper valley wine grape field well and a central valley observation well. After dropping during the winter, water tables increase by 40 cm three to four weeks after the beginning of irrigation channel operations in September (Figure 8a). Around late spring and early summer, water levels drop again. More detailed information on groundwater response to irrigation seepage was observed with the automated sampling locations in the upper valley wine grape field (Figure 7b). Both wells showed a 35 cm water table increase after the opening of irrigation diversions (usually after September 20th), with the timing of the increase related to their distance from the irrigation canal. Well 1 was located 110 m from the irrigation canal and the water table peak was observed 22 days after the opening of irrigation diversions. Well 2 was located 390 m from the irrigation canal and the water table peak was observed 28 days after the opening of irrigation diversions.

Figure 8: Irrigation seepage and groundwater recharge patterns.

Ongoing process-level study (Arumi, unpublished data) combined with basic water balance concepts lead to the following interpretations of these seepage and groundwater patterns. At the annual scale, water levels drop until June because there is not recharge from precipitation or runoff. There is a delay in water table rise after the beginning of the irrigation season corresponding to the time required to replenish soil available water capacity. Water table declines in spring and early summer correspond to the period of increased plant evapotranspiration demand. For the detailed period before and after the opening of irrigation diversions, the delay of about 5 days from the water table peaks in well 2 to well 1 is a function of the travel time of the pulse of seepage water moving from the irrigation canal under crop lands. Both wells lie on the same line perpendicular to the irrigation canal. Though the peaks (Fig 8b) represent different travel times of seepage pulses, the water table decline corresponds to the exact same period at different locations because it is caused by the evapotranspiration demand.

In the absence of a complex irrigation network, the recharge processes in the Peumo Valley and similar valleys in the region would involve winter wet season surface and subsurface runoff from the Coastal Range uplands into the valley floor. This runoff would recharge groundwater in winter, leading to higher water tables. In the summer dry season, plant evapotranspiration demand and lack of precipitation or upland runoff would lead to lowered water tables. However, in the Peumo valley where there is now a highly developed irrigation canal network, the large main irrigation canals bound the edge of the valley, intercept upland runoff, and transport the water to the river at the lower end of the valley. This runoff interception by canals greatly reduces valley aquifer recharge by upland runoff. The irrigation canal seepage serves to recharge the shallow aquifer in summer, unlike natural recharge that would take place in winter, leading to higher water tables in late spring (Figure 9).

With land and water managers considering lining irrigation canals, it is important to consider how interactions between surface water and groundwater may be affected by reducing or removing canal seepage. In the current water balance of the valley, shallow groundwater levels depend on recharge from irrigation canal seepage. Modifications to the canals, such as lining with impermeable materials, could lead to a reduction of groundwater recharge and changes in crop production patterns due to lowered groundwater levels.

Rain

Rain

Groundwater

Figure 9: Conceptual scheme of rainfall-runoff-recharge processes

Groundwater

Figure 9: Conceptual scheme of rainfall-runoff-recharge processes

With land and water managers considering lining irrigation canals, it is important to consider how interactions between surface water and groundwater may be affected by reducing or removing canal seepage. In the current water balance of the valley, shallow groundwater levels depend on recharge from irrigation canal seepage. Modifications to the canals, such as lining with impermeable materials, could lead to a reduction of groundwater recharge and changes in crop production patterns due to lowered groundwater levels.

Also, in terms of ecological functions supported by this agricultural landscape, riparian vegetation along the irrigation canals play an important role in diffuse source pollution control and wildlife habitat, both aquatic and terrestrial (Cey et al., 1999; Cirmo & McDonnell, 1997). Field campaigns identified high riparian vegetation production along irrigation canals in the Peumo Valley, and foliar analysis of this riparian vegetation showed high nitrogen concentrations (1000 mg kg-1) similar to local fertilized wine grapes (Arumi, unpublished data). Accordingly, the riparian vegetation acts as a biofilter that naturally contributes to maintenance of lowered nitrogen concentrations in surface waters. It is likely that the riparian vegetation contributes to improved water quality by reducing turbidity and conductivity, important for agricultural management and aquatic habitat, though further studies are necessary to fully characterize these functions.

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