# Remote Sensing for Irrigation of Horticultural Crops
## 4 Approaches to Precision Irrigation
### Infrared Thermography
#### 4.2.2 Time-Temperature Threshold (TTT)  
The TTT is a method based on a canopy temperature threshold and a time threshold [187; 188]. Because it is a response method of automatic control, the TTT method does not need supplementary inputs for activating irrigation; and it has been credited for regulating water-use efficiency. [PERSON] et al. [188] found no statistical differences in yield and WUE, when comparing TTT and with manually irrigated plots, in soybean and corn.  
[PERSON] et al. [189] studied the relationship between cotton irrigation scheduling, using soil water measurements and two thermal indices. An automatic irrigation scheduling determined by a TTT algorithm reduced applied water volumes that did not negatively affect yield. In well-watered conditions (full irrigation), the TTT method increased WUE with respect to the water balance method; differences were not significant at reduced water depths (rates of 33 and 67% of full irrigation). The authors also found similar trends between two thermal indices, although the daily theoretical CWSI resulted in over-irrigation when it was over 0.5.  
[PERSON] et al. [190] compared the TTT and ET-based irrigation scheduling for corn production. The TTT method accumulated time when the crop was above a specified temperature. An irrigation event signal was triggered when the accumulated time exceeded the specified TTT threshold. The authors compared (i) three irrigation treatments with TTT values of 2.5 h, 4.0 h or 5.5 h above a canopy temperature of 28 \({}^{\circ}\)C; and (ii) two \(\mathrm{ET_{c}}\) treatments (100% and 65%), plus a treatment receiving no further irrigation after mid-July in Kansas. The results indicated that a TTT with a threshold of 2.5 h corresponded to a fully irrigated treatment.  
[PERSON] et al. [191] developed a method of integrating the CWSI over a day for presenting clear CWSI signals occurring in some environmental conditions, near solar noon. CWSI-TT was the accumulated time when CWSI was greater than a threshold value of 0.45; this cumulated time value was compared with a CWSI-TT value obtained for a well-watered crop. The use of CWIS-TT, however, can lead to an over-irrigation due to false positive triggers that may be generated early in the season. CWSI-TT provided yield results similar to scientific irrigation scheduling using a neutron probe.  
[PERSON] et al. [192] tested an automatic control system along with a wireless network of soil, thermal and weather sensors, developed for an apple orchard. Seven irrigation-scheduling algorithms were tested: TTT, CWSI, soil-based using granular matrix sensors (SOIL), an ET model, a soil water balance (WB), a combination of SOIL and WB, a conventional irrigation practice used in the region, and soil-based using a neutron probe (NP) as benchmark. The treatments TTT, CWSI, NP and WBsubstantially reduced water (70%) while maintaining \(\psi_{\text{stem}}\) within the non-stressed range. WB seemed to demonstrate the characteristics of being economical, easy to implement and accurate.  
[PERSON] et al. (2019) conducted an experiment on corn to evaluate several water stress indices based on canopy temperature. All thermal indices responded quickly to irrigation-treatment differences at major growth stages, in particular Degrees Above Non-Stressed index and Degrees Above Canopy Threshold index were highly correlated with CWSI above the corn threshold of 28 \({}^{\circ}\)C used in the TTT method. Furthermore, all indices showed a linear relationship with soil water deficit at high temperatures. Degrees Above Canopy Threshold was the best index due to (i) the strong relationships with other indices and crop water measurements; and (ii) because it was derived from a single canopy temperature measurement.