Use of Different Models to Estimate Plant Water

Overview

As industrial emissions and human activities are greatly contributing towards the warming of the climate, the Intergovernmental Panel on Climate Change (IPCC), in its fifth assessment report concluded that about 95% of probability is that human activities over the past 50 years have warmed our planet (Bajracharya, Mool et al. 2008). Changing climatic conditions causes major issues in many economic sectors, including agriculture. Global warming has resulted in increased intensity and severity of heat waves, frequency of droughts and intensity of precipitation events.

These changes are already and will impact the growth and yield of crops (the United States Environmental protection agency, 2017, available at https://www.epa.gov/). Agriculture is exquisitely sensitive to changes in climatic patterns. Drought conditions at the critical growth stages of the crop and floods at the ripening or harvesting stage results in the decrease of yield or sometimes lead to the death of the crops.

 With prolonged drought conditions and overuse of water, the water table has been recharging at a slower rate than usage, which results in a shortage of the water supply. Due to urbanization and population expansion, water use has been increasing thus results in reduction of the irrigation supplies in the future (Sammis, Gutschick et al. 2013)). Irrigation management plays a key role in developing sustainable agricultural techniques. To feed world’s growing population, different types of agricultural commodities needs to develop but also with a minimum of environment footprints (Fernández, Green et al. 2008). For fulfilling these demands, efficiency of agrochemical and water needs to be https://www.epa.gov/ enhanced. Farmers increase the water use to cope up with the risks due to drought conditions rather than applying irrigation when plants needs irrigation.

 Changes in climatic conditions couples with increasing competition for water, suggests that studies related to increasing WUE or assessment of water use by plants is becoming more important to understand the various plant physiological processes and their regulatory processes. Various environmental and biological variables are required to predict water use by the plants which is relatively a complex process. Evapotranspiration (ET), a loss of water from soil as well as from plants or trees, is driven by meteorological conditions and also soil moisture content at the root zone. If there is a sufficient soil-water content, ET will be directed by the leaf area. Water stress in the plants can also be estimated by calculating crop-coefficient which is a ratio of actual evapotranspiration to potential evapotranspiration calculated by using actual weather data using Penman-Monteith equation or by using reference evapotranspiration values of particular crop from FAO-56. Thus, Et can also be calculate by using product of reference ET and a crop coefficient (recommended by FAO). Crop Coefficient values do no alter much with varying crop characteristics and a little due to climatic variations. Therefore, it can be used by calculating at one location and transfer among other locations and climates

Body

Definitions and factors affecting irrigation scheduling Irrigation Scheduling defines when, how much and how often irrigation water has to apply to the plants. How much and how often water has to be applied depends on the water needs of the plants. Irrigation scheduling depends on the soil water measurement and soil moisture content indicates the irrigation need the plant. Soil moisture content can be estimated by subtracting the inputs (irrigation and rainfall) and losses (evapotranspiration, runoff, drainage) of water from soil. Main inputs of irrigation scheduling approaches are as follows:

1. Soil water measurements
2. Soil Water potential: Measured by using tensiometer, piezometer, psychrometer
3. Soil water content: measured by using TDR, neutron probe, soil moisture sensors

Plant Stress Sensing

Physiological responses: measured using calculations from stomatal conductance (porometer, thermal sensing, sap flow sensors) and by estimating growth rate. Soil water has both direct and indirect impact on the plant growth and yield of the plant as water stress especially at a critical stage will lead to decline in yield and production of a poor quality nuts. Goal of irrigation should not be only to maximize the production but also reduce the environmental impacts irrigation or to maximize the production even when there is a water scarcity.

Various methods of measuring transpiration The amount of water transpired by a plant provide estimation for irrigation control, biomass production and to be familiar with plant-water relations. Thus, there are several techniques that are beneficial to understand the water use by plants which further provide help in irrigation scheduling or in designing irrigation practices. Several plant-based methods had been developed by various scientists for estimation of plant water use along with few drawbacks like variation in different plants and problem of providing exact threshold values of different crops. But some of these issues are resolved by advancing scientific knowledge about electrical sensors. Moreover, advancement in remote-sensing methods along with Plant-based methods have provide benefits in scheduling irrigation even in heterogenous crops. 

A variety of sensors including sap flow sensors, trunk diameter variations, leaf turgor pressure, water content in the trunk, canopy temperature, electric water potential, eddy covariance system ( two sensors, gas analyzer and sonic anemometer) and whole-canopy gas exchange, a direct method of measuring transpiration can be used to measure transpiration in different plants. Sap flow sensors used to estimate the transpiration rate even with a small-scale heterogeneity on the basis of age, size and density of plants. Sap flow along with eddy covariance technique will provide even short time fluctuation of sap flow and atmospheric momentum, temperature and air humidity. It will also help to estimate the evaporation from the surface of the trees along with individual tree transpiration whereas if whole tree transpiration including branch and stem is required, whole canopy-gas chamber is beneficial. But it alters the microclimate of the plants thus, sap flow sensors are portable and are used without any disturbance.

Use of various models for estimate of water-use According to some studies, it has been believed that a slight water stress faced by a plant will lead to improvement of partitioning of carbohydrates more towards reproductive parts like fruits as compared to vegetative parts thus, helpful in controlling excessive vegetative growth. This process is known as regulated deficit irrigation (RDI). But an accurate soil moisture values are required for successfully implementation of RDI. To maintain water stress in a certain tolerance range is quite difficult and failure to maintain it will lead to decrease in yield and quality of the produce. So, if higher precision is required to schedule irrigation, method of irrigation should be modified from flood irrigation to micro irrigation methods. Plant-based methods are more advantageous on the basis of measuring plant’s responses to different environmental conditions. Therefore, the output from these models, provide output to generate the estimate of stress faced by the plants.

Direct methods:

1. Physiologically based models: An empirical model based on stomatal conductance had been developed by a Physiologist especially in fruit trees with a decoupling factor where transpiration is effectively controlled by stomata. However, these models hadn’t incorporated the effect of water stress. Along with this, plant and soil hydraulic conductivity were completely ignored which were important to modelling because soil and xylem conductivity decreases under hydraulic tension.
2. Root water uptake models: HYDRUS 1-D model is a numerical model and was developed by Šim?nek, van Genuchten et al. (2008) to simulate water, heat and solute movement in saturated-unsaturated media. Simulations for water movement, heat and solute has been done for several tree crops using HYDRUS 1-D model. In HYDRUS 1-d, Soil water flow is modeled by using Richard’s equation.

Indirect methods:

Several indirect methods can also be used for measuring water stress inside the plants, but these methods have certain disadvantages. Stem and fruit diameter, Sap flow, xylem cavitation, thermal sensing, automation, and leaf thickness can be measured to measure water content of the plant indirectly.

Conclusion

Initially various sensors are used to estimate the transpiration of the trees but soil factors are completely ignored by that methods. Plant based methods have certain advantages including greater relevance to plant functioning but there are certain practical difficulties of using these methods as compared to soil based methods Thus, use of plant based methods understanding the mechanism inside the plants and soil will be beneficial to do irrigation scheduling in plants which will results in making different prediction under different irrigation and weather scenarios and leads to help the growers to produce more and more with good quality. If still in any case plant based methods have to use, we have to think about the plant measurements which must be a true representatives of the water stress inside the plant.

References

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