Recycling irrigation reservoirs (RIRs) have been adopted in production nurseries to conserve increasingly scarce and costly fresh water resources. Such practices help ensure the availability of water for plants during periods of limited rainfall or at critical times during plant development. The quality of the recycled irrigation water is a significant issue for this conservation practice, as it could impact pathogen survival, water treatment efficacy and other agricultural practices.
A number of recycled irrigation water quality parameters must be considered because of their impacts on plant quality and productivity. Increased electrical conductivity (EC) decreases the growth of Ranunculus asiaticus
] and other crop plants, such as beans, onion, corn and potato [2
]. Water pH affects nutrient solubility and availability to container grown plants. For example, the solubility of phosphorus, iron, manganese, zinc, and boron decreases dramatically with increasing pH from 5.0 to 7.0 and becomes very limited in alkaline water [3
]. Phosphorous becomes less available to the plants when pH is above 7.2 [3
]. Water temperature (T) [4
], pH [10
], dissolved oxygen (DO) [11
], EC [12
] and oxidation-reduction potential (ORP) [13
] also affect the survival and growth of plant pathogens in the same reservoirs.
In light of the importance of water quality to crop health and productivity, it is desirable to monitor water quality parameters, and define trends in RIRs. Such information is important to decision-making on water treatment such as acidification, aeration to bring individual parameter to ranges suitable for crop health and production. Water quality monitoring programs have been carried out in natural lakes, reservoirs and rivers to document the spatial and temporal variations in the hydrochemistry of surface waters [14
]. The surface water quality can change daily, monthly, seasonally, or annually resulting from agricultural activities, land use types and rain events [17
] as well as due to biogeochemical processes, atmospheric deposition, and hydrological changes. Researchers have evaluated freshwater quality spatial variation using multivariate techniques including cluster analysis (CA), principal component analysis (PCA), and discriminant analysis (DA) [16
Water quality in RIRs and its spatial variation has not yet been extensively studied. Hong et al
] conducted continuous water quality monitoring in RIRs in the Mid-Atlantic region of the U.S. since 2006, documenting water quality fluctuation in RIRs. However, the continuous data was collected at a single point and depth in each RIR. Zhang et al.
] reported that RIR water quality varies vertically in the water column due to thermal stratification. Whether and how water quality in RIRs may vary between points within the same reservoirs is unknown.
The goals of this study are: (1) to determine water quality variation with respect to spatially separated sampling points in RIRs and determine whether the variation is significant; (2) to document the pattern of spatial variation of water quality across different seasons and nurseries; and (3) evaluate the potential impact of the variations on water quality monitoring in RIRs.
This study examined the spatial variation of nine water quality parameters in recycling irrigation reservoirs, an aquatic environment of great importance to agricultural production and water resource conservation. Water quality differences exist between the entrance and middle of RIRs for several parameters. T and ORP were mostly lower at the middle than at the entrance, while DO, pH and chlorophyll a were mostly higher at the middle than at entrance. EC, salinity, TDS and TUR had very small differences between sampling points. Generally, the two-point differences were small in magnitude for all parameters, even though they were statistically significant. It should be noted that water quality samples from the entrance were taken at times when no runoff was flowing into the RIRs. The chemical gradients between the entrance and the middle of the RIRs may be diminished due to physical mixing. Additional analysis of outlet data shows that the range and magnitude of water quality difference between the middle and the outlet are comparable to those between the middle and entrance of RIRs.
The water quality differences between the middle and entrance varied with seasons and geographic location. The water quality differences were greater from April to October when water column was stratified than the non-stratification periods from November to March [22
]. The differences also varied with geographical location and time of year as indicated by cluster analysis. Significant clustering due to geographical locations and time of year was evident yet not consistently distributed in hierarchical placement. High similarity was observed between MD11 and MD21 and also between VA12 and VA21 in 2013. Temperature differences between sampling points among different reservoirs are significantly correlated with DLI, wind speed and precipitation (p
< 0.05), and pH differences are significantly correlated with precipitation, while other parameters are not, which indicate weather conditions have limited contribution to grouping of VA reservoirs and MD reservoirs. The scattered aggregation of the grouping indicates other covariate factors not identified in this study may still need to be elucidated. The clustering of reservoirs may also be due to the irrigation activities in each nursery which has not been documented in this study.
Water quality differences between the entrance and middle of RIRs, although statistically significant, are not large enough in magnitude to cause significant impacts on plant pathogen survival. A variety of plant pathogens including bacteria, fungi and oomycetes are present in the RIRs [23
], among which Phytophthora
species are the most economically important to nursery production. Phytophthora
species, such as P. gonapodyides
and P. pini
have been reported to survive best at temperature 20–25 °C [9
] and a few high-temperature tolerant species, such as P. aquimorbida
, P. hydrogena
, P. hydropathica
, P. insolita
, P. irrigata
, and P. virginiana
have an optimal growth temperature of 35 °C [4
]. Temperature differences between these two sampling points of within 1 °C would not alter the temperature biome for pathogen activities. Zoospores of several Phytophthora
species including P. megasperma
, P. nicotianae
, P. pini
and P. tropicalis
were found to survive best at DO concentrations in a very narrow range between 5.3 and 5.6 mg/L [11
] and the majority of DO concentrations in RIRs are outside this range. DO differences were within 2 mg/L for most sampling dates, which may not change DO conditions in RIRs from being unfavorable for pathogen survival. Survival rates for zoospores of seven Phytophthora
species were pH dependent [10
]. Most pH differences were within 1.0 unit in RIRs and would likely to have little effect on pathogen survival. Three high-impact quarantine pathogens P. ramorum
, P. alni
and P. kernoviae
survive better at EC levels greater than 1.89 dS/m than at EC levels below 0.41 dS/m [12
]. The EC levels and differences were primarily less than 0.41 and within 0.01 dS/m, respectively. Such small variations would not alter the negative influence on zoospore survival. ORP values above 650 mv can inactivate human pathogens [13
]. Although the ORP variation is mostly within 100 mv, the ORP ranges in RIRs are still less than 650 mv, which would have a limited impact on pathogen inactivation. Compared to the ranges of each water quality parameter that affect pathogen survival, the absolute water quality differences between two sampling points were relatively small, which may not alter water environment conditions and pathogen survival.
Water quality differences between sampling points in RIRs could impact chlorination efficacy. Chlorination, widely used to disinfect water, performs best at pH 5.0 to 6.5 [25
], and increasing pH would diminish the performance of chlorine. The efficacy of chlorination is approximately 100% at pH 5, 96% at pH 6, 75% at pH 7, 26% at pH 8, 10% at pH 9 and 6% at pH 10. The pH differences, even though it was within 1.0 unit, could greatly affect the performance of chlorination. Thus, it is important to take pH measurements at the point where water is treated.
The spatial variation of some water quality parameters could affect plant growth and productivity. The solubility of some micro- and macronutrients decreases dramatically within 2.0 unit of pH [3
]. The desirable pH level for some plants is between 4.5 and 6.5 [26
]. The pH differences observed in this study could change the availability of nutrients to plants, depending on the buffer capacity. The recommended substrate EC levels for most landscape plants are between 0.5 and 1.0 dS/m or 0.8 and 1.5 dS/m depending on the type of fertilizer applied [26
]. The EC differences within 0.01 dS/m are unlikely to affect plant growth due to the scale.
Single point monitoring in RIRs is sufficient for most water quality parameters except pH; and such monitoring scheme could save growers substantial cost and time compared to multi-point monitoring. Water quality data are essential for grower to make an informed decision on water treatment before water is delivered to crops. Water quality monitoring equipment can be expensive and taking measurements is time consuming. Our results demonstrate that two-point differences of most water quality parameters except for pH do not have significant impacts on pathogen survival, water disinfection and nutrient availability.
The outputs of this study also could be utilized in RIR water quality dynamics modeling that is important to horticultural industry. The modeling is essential to identify kinetic processes controlling water quality fluctuations in RIRs and project water quality changes temporally and spatially. This study provides the spatial variations of water quality parameters, which is important in determining the spatial resolution of water quality modeling and data inputs.
Overall, this study characterized the spatial variations of nine water quality parameters in RIRs, an emerging aquatic environment. These data provide a basis for considering the computer simulation of water quality dynamics and plant disease management in RIRs. The two-point water quality differences of most water quality parameters except pH, although statistically significant, are not large enough to cause significant impacts on crop health and productivity. This study suggests that single-point monitoring in RIRs is sufficient for most water quality parameters except pH. Additional pH measurements are necessary to made decisions on water treatments before water is delivered to crops. More studies on temporal variations of water quality, reservoir mixing (i.e., aeration) and reservoir geometry (i.e., size and depth) are warranted to develop further recommendations on management of water withdrawal, water disinfection, fertigation, and algal blooms.