Monitoring, Modeling, and Managing soil-water-disease interactions in citrus trees of central India

Characterizing the eco-hydrological processes and associated interactions within the soil-plant-atmosphere continuum (SPAC) pose formidable challenges to agronomists, irrigation engineers, plant physiologists, and water managers. These processes include: i) water and nutrient uptake by the plants that is influenced by root morphology and soil moisture availability, and ii) water lost to transpiration vis-à-vis carbon gain to photosynthesis that are influenced by plant phenology, stomatal conductance and meteorological conditions. Vidarbha region in Maharashtra, India is the largest producer of mandarin oranges (Citrus Retuculata) accounting for about 40% of country’s production with a yield of 6 t/Ha, much lower than nation’s average of 9.2 t/Ha. Low citrus crop production is primarily attributed to improper management activities practiced in the region that result in the formation and propagation of a water mold disease called root rot (Phytophthora spp.). The disease originates in the rhizosphere and progress towards the trunk with symptoms seen at the surface at much later time, thus making difficult to implement management practices. A physical understanding and critical assessment of soil-water-disease interactions can help in managing the disease and improve citrus crop productivity, with available resources. This research is aimed at a comprehensive understanding of eco-hydrological processes happening between rhizosphere and planetary boundary layer for the citrus trees of Vidarbha region. The specific objectives of this research include: i) Understand the hydrological and plant controls on root water uptake (RWU) mechanism dominated by health of citrus tree ii) Develop disease stress response functions and incorporate into a variable saturated flow model to simulate RWU from healthy and disease citrus trees iii) Develop single and dual crop coefficient curves for citrus trees viii iv) Characterize water use efficiency (WUE) of citrus trees of central India at ecosystem to regional scales The first objective is achieved by simulating RWU using existing numerical tools and validating with experimental observations. Two research plots, one around a healthy mature and the other around a diseased mature (Phytophthora spp. affected) tree were considered. Three-dimensional electrical resistivity tomography (ERT) performed at the two locations revealed that the soil moisture profiles following irrigation are different between the two plots. A twodimensional axisymmetric form of Richards’ equation was then solved using HYDRUS (2D/3D) with an empirical sink term representing RWU. A global sensitivity analysis was performed to identify the root distribution parameters that influence soil moisture simulations. These parameters were then optimized using a genetic algorithm for healthy and diseased conditions. It was observed that the diseased orange tree had consumed less water, leaving high soil moisture in the rhizosphere, a condition favorable for the further growth of disease causing fungi. Results also conclude that the error in simulating RWU from a disease tree by ignoring the health condition significantly high, from the existing numerical models. Following the fact that, uptake mechanism in response to irrigation is contrasting between healthy and disease trees, second objective aims at developing a disease stress response function, and incorporate into variable saturated flow models. Field experiments were performed at four citrus trees with varying disease intensities (on a scale of 0 to 4) and hydrological fluxes were accurately monitored following irrigation events. Uptake reduction due to disease stress is modeled using piece-wise linear and S-shaped nonlinear models, and parameterized by defining thresholds on propagule count (PC) of inoculum bacteria for each crop growth stage. The developed functions were implemented with a numerical implicit model (HYDRUS 2D/3D) to simulate water uptake from a root-system in symmetric radial flow domain. The proposed model was successfully tested for soil moisture and plant transpiration fluxes ix under various water and disease limiting conditions. Results conclude that, calibration targets to validate uptake reduction functions should be chosen dynamically based on the dominant stress experienced by the root system. Stakeholders of the region are concerned of adopting efficient management practices in response to changes in hydro-climatic regime. For this, it is imperative to understand crop water requirements during citrus growth cycle to suit local agro-climatic conditions. To achieve this, region specific citrus crop coefficients were developed using water balance and energy balance approaches. Reference evapotranspiration was estimated using PenmanMonteith method for the period Jan-Dec 2017. ET fluxes derived from eddycovariance (EC) technique were used to develop single crop coefficient curves at daily, weekly, and seasonal scales. Site-specific Kc values for initial, mid, and late season were found to be 0.43, 0.78, and 0.80 respectively. ET partitioning was done by estimating soil evaporation coefficient (Ke), and basal crop coefficient in the presence of water stress (Ks × Kcb) using soil-water balance (SIMDualKc) and EC flux partitioning (EC FP) methods. Energy based flux partitioning was done by considering the correlation between high-frequency water vapour and carbon fluxes and applying flux variance similarity principles. Direct measurement of evaporation (E) and transpiration (T) at four citrus trees (using micro-lysimeters and sap flow meters) was used to assess the performance of two models. Three-stage basal crop coefficients from SIMDualKc and EC FP methods were found to be 0.18, 0.57, 0.63 and 0.26, 0.51, 0.59 respectively. Low crop coefficients during initial stage are attributed to the intentional water stressing of the crop, to initiate blooming. Following an accurate assessment of crop water demand, it is necessary to improve crop productivity with manageable resources. WUE is the key ecohydrological trait that accurately relates water and carbon fluxes between vegetation and atmosphere. Dynamics of WUE across multiple time scales were analyzed to understand the response of citrus ecosystem to natural and anthropogenic changes. EC measurements were used to measure ET and gross primary product (GPP) fluxes using two crop cycle data (2016-17). On a daily x scale, ET and GPP have recorded similar trends with peak occurring during fruit development stage. Daily WUE ranged from 0.22 to 3.39 g C kg-1H2O, with a mean of 1.77 g C kg-1 H2O. Inter-seasonal variability in WUE has emphasized the need for partitioning of fluxes between the growth stages. LANDSAT images were used to extrapolate and scale-up the tower-based measurements to characterize spatiotemporal variability in WUE at regional scale. Eight biophysical indices derived from LANDSAT were then regressed with WUE estimates, to see if these indices either in solitary or in combination can explain the WUE dynamics for citrus orchards. Results conclude that WUE has a strong dependency on enhanced vegetation index (EVI) and soil-adjusted vegetation index (SAVI) with varying correlation strengths during the crop cycle