Figure5.6shows the evolution of the soil matrix potential for all simulations with an irrigation depth of 25 and 100 liters, the scenarios with a depth of 50 liters are shown in appendixA.5.
CHAPTER 5. RESULTS OF HYDRUS 2D SIMULATIONS T. Müller
Threshold −5 [kPa] Threshold −10 [kPa] Threshold −15 [kPa]
Threshold −20 [kPa] Threshold −25 [kPa] Threshold −30 [kPa]
Threshold −40 [kPa] Threshold −50 [kPa] Threshold −100 [kPa]
−100
Sensor depth 5 [cm] − Irrigation depth 25 [L]
Threshold −5 [kPa] Threshold −10 [kPa] Threshold −15 [kPa]
Threshold −20 [kPa] Threshold −25 [kPa] Threshold −30 [kPa]
Threshold −40 [kPa] Threshold −50 [kPa] Threshold −100 [kPa]
−100
Sensor depth 5 [cm] − Irrigation depth 100 [L]
Figure 5.6: Evolution of the simulated soil matrix potential at various depths for all scenarios during the early growth stage and using an irrigation depth of 25 liters and 100 liters.
For all scenarios it appears clearly that the soil matrix potential below 15 cm does not uctuate with time. Even at 15 cm the evolution of the potential hardly uctuates, except for the last scenarios with an irrigation depth of 100 liters and a threshold lower than -50 kPa. Indeed, the roots at that stage only reach a depth of 10 to 15 centimeters, so that the water uptake is very limited below 10 cm. Moreover, depending on the threshold, the irrigation either saturates the whole soil (high threshold), so that the
CHAPTER 5. RESULTS OF HYDRUS 2D SIMULATIONS T. Müller
potential stays very high, or when the soil is drier only the upper part of the soil is recharged by the low irrigation depth. The gure A.9in appendixA.5shows the mean soil matrix potential over time at dierent depths. The drop that occurs at a certain depth corresponds to the edge of the wetted bulb. It shows that with a depth of 25 liters, only the rst 10 cm are reached by the irrigation front, for the 50 liters scenarios the depth is about 13 cm and for the 100 liters scenarios the depth is 170 cm.
The rst conclusion that can be done is that placing the sensor below 10 cm is not recommendable as the sensor will only measure low variations and the placement will not be representative of the water availability in the root zone.
This represents a major limitation of our irrigation management system. If we want to control irri-gation using only one sensor during the whole growth, a maximal depth of 10 cm is required for the sensor. This depth seems however justi ed as it corresponds also to the depth of maximal root density from our root analysis (see gure4.12from chapter4.2.4).
The gure 5.7 shows the reduction of transpiration depending on the threshold used, as well as other relevant results.
Threshold at 5 cm depth [kPa]
Transpiration reduction [%]
Mean root zone soil matrix potential [kPa]
Transpiration reduction [%]
Threshold at 5 cm depth [kPa]
Mean root zone soil matrix potential [kPa]
●
Threshold at 5 cm depth [kPa]
Irrigation / ETc [%]
Fluxes with sensor at 5 cm depth
Figure 5.7: Summary of all scenario results for the early growth stage and given the irrigation depth. The upper-left plot represents the simulated cumulative actual transpiration volumes over the cumulative potential transpiration amounts given the irrigation threshold used at 5 cm depth. The bottom-right plot shows the relationship between the mean root zone soil matrix potential over time and the soil matrix potential threshold used. The upper-left plot shows the ratio of transpiration reduction given the mean root zone soil matrix potential.
The bottom-right plot represents the reduction in irrigation water applied using the ratio of cumulative irrigation amounts over the cumulative amounts of potential transpiration and evaporation given the threshold used.
For the early growth stage, it has been discussed that water stress could already aect plant growth before transpiration actually starts to decrease. In our models, it was shown from the calibration that the transpiration reduction function overestimated the actual transpiration reduction (but not the local root water uptake reduction), so that we consider the transpiration reduction as a good indicator of water stress.
For this early stage, we propose to x the optimal threshold just before transpiration starts to decrease.
Looking at the rst plot of gure5.7, with a sensor at 5 cm, this threshold is -20 kPa with an irrigation depth of 25 liters, -30 kPa for 50 liters irrigation depth and -40 kPa for 100 liters. When looking at the last plot, we observe that the ratio of irrigation over evapotranspiration is strongly reduced for high thresholds and then presents a very at slope. The proposed threshold are located near at the start of this plateau, con rming that the threshold seem optimal for the purpose of both water stress avoidance
CHAPTER 5. RESULTS OF HYDRUS 2D SIMULATIONS T. Müller
and water savings. For more security, the threshold could be increased a bit, and placed exactly at the beginning of the plateau, about 5 kPa higher. If we want to place the sensor at 10 cm depth, the threshold should be adapted. The table5.3summarizes the selected thresholds.
Irr. depth Threshold at 5cm Threshold at 10cm Irrigation/ETc Irr. frequency
[L] [kPa] [kPa] [%] [1/day]
25 -20 -20 40 1.5
50 -25 -20 39 1/1.3
100 -30 -15 42 1/2
Table 5.3: Summary of the selected optimal thresholds and irrigation schedules for the early growth stage based on all HYDRUS scenarios.
It is interesting to note, that at deeper depth, the threshold must be lower when the irrigation depth is higher, due to the higher recharge of the soil after an irrigation event.
The irrigation depths selected for our scenarios do not lead to signi cant dierences in terms of wa-ter savings compared to wawa-ter stress. One hypothesis of the study was that increasing the irrigation depth and lowering the irrigation frequency would save water as the soil surface is less frequently wetted, limiting the top soil evaporation. This conclusion was proposed by Mermoud et al., 2005 [50] who used HYDRUS 1D to simulate surface irrigation in semi-arid zones. The model has shown that there was no clear dierence in evaporation reduction between the scenarios for triggered irrigation. Since the soil is only partially wetted (in contrast to surface irrigation), the reduction of evaporation by decreasing the irrigation frequency is compensated by an increase of the wetted diameter at the soil surface, due to the higher irrigation depth. The evaporation reduction was in the end very similar for each irrigation depth.
Irrigating with a higher depth increases the depth of the wetted bulb, which may promote a deeper root system, however it reduces the irrigation frequency and eld experiments have shown that a higher frequency slowed the plant growth.
From the simulations, using a very low irrigation depth seems to hardly reach 10 cm which is probably not optimal.
To conclude, we propose to use an irrigation depth corresponding to 50 liters, with a threshold of -20 kPa at 10 cm depth and located about 5 cm away from the stem.
Our simulations show that great water savings can be achieved at that early growth stage. A part of the water savings are directly linked to the drip kit system since a part of the soil is not irrigated, limiting the evaporation ux, which is the major source of water loss. Controlling irrigation shows that greater water savings can be made by avoiding leakages and controlling the soil moisture at the soil surface. It should be noted that in these simulations we use a value for the crop coe cient Kc corresponding to its maximal value (around 1.2) and not the mean value proposed by the FAO, which already takes into account evaporation reduction, but which can vary greatly depending on the irrigation frequency. As a result the calculated ETc values to its maximal value and the ratio Irrigation / ETc appears thus lower.