Project

Transpiration rate of cocoa trees in Indonesia

Water use by cocoa trees is poorly understood, yet important for yield prediction and on-farm water management. This project aims to measure transpiration rates in cocoa fields in Indonesia. The work involves (1) taking leaf and environmental measurements, (2) using energy balance equations to solve for leaf-level transpiration rates, (3) upscaling to canopy level, and (4) comparing these values with alternative measurements of transpiration rates. The finalized measurement protocol will be used to measure transpiration rates at several field sites.

Project description

Water use by cocoa trees is poorly understood but highly important for stable cacao production and yield. Quantifying cocoa tree water uptake with respect to atmospheric and soil conditions will help to improve the understanding of water dynamics in cocoa, while also providing direct benefits in terms of irrigation management, drought stress management, and yield forecasting.

Various methods exist for quantifying canopy-level transpiration rates: the “gold standard” is to use gravimetric methods with weighing lysimeters. Several in-field methods can alternatively be applied to measure or infer transpiration rates. One family of such inference measurements relies on taking measurements of leaf and environmental variables and using the energy balance to solve for the latent heat of evaporation, and scaling up to canopy level based on leaf area index. Another method is to measure sap flux density and scale this to tree level (sapflow measurement).

Glasshouse and field experiments have been established by Mondelez International at sites in East Java, Indonesia. New technologies are tested in a commercial setting on a large cocoa estate on an island in the Maluccas.

This project aims to refine and test a protocol for in-field determination of
canopy-level transpiration rate in cocoa trees in Indonesia.

Objectives and methods

This project aims to determine canopy-level transpiration rate in cocoa trees in Indonesia. It will involve:

Testing and refining a simple protocol for estimating transpiration rate based on a leaf energy balance model and data recorded from: a leaf porometer, leaf temperature sensors, radiation sensors, wind speed and humidity sensors; this will be conducted with potted trees in glasshouses in Indonesia.
Applying this protocol to conduct field measurements in an existing experiment in East Java, comparing findings with data from sapflow rate experiments (already in place) and changes in measured soil water content.
Applying the field-validated protocol to measure transpiration rates on a commercial cocoa estate.

Expectations

It is expected that a reliable set of measurements of canopy-level (i.e. per-tree or per-ha) transpiration rate will be produced, along with a solid protocol for conducting in-field transpiration rate measurements in cocoa trees in Indonesia.

The thesis will include: a review of past efforts to measure transpiration rates in cocoa; a description of the theoretical bases for calculating transpiration rate in different ways (gravimetric, water balance, energy balance, sapflow); a description of the methodology followed to optimize the in-field transpiration measurement protocol; a description of the finalized protocol; results of measurements taken at the different field sites using the protocol; a discussion of how these results compare with previous efforts; and recommendations for future work.

Required skills

We are looking for an enthusiastic and creative student with an interest in water use and tropical agriculture. Some demonstrated knowledge of agro-meteorology and evapotranspiration processes and models will be helpful, but not essential.

The student will also need to work with a large team in Indonesia - so must be happy to travel, and be respectful of cultural differences. The work will involve some physical work, including climbing ladders to get to upper leaves (±4 m height).

Types of research/work

This project is research-based, and involves: (1) desktop research; (2) taking measurements within cocoa tree canopies; (3) conducting relatively simple data analyses.

Period

We are hoping that transpiration rate measurements can be taken during both dry and rainy periods in Indonesia. For this reason, we expect that travel to Indonesia should span at least three months, during the period July-October 2024. Due to preparation, thesis should start in May 2024.