Soil Water Dynamics, Effective Rooting Zone, and Evapotranspiration of Sprinkler Irrigated Potato in a Sandy Loam Soil

Abstract

Potato (Solanum tuberosum L.) is a very sensitive crop to water stress and timely irrigation water management improves tuber yield and quality. The objectives of this study were to (1) investigate soil water dynamics under potato crops across their root zone and (2) estimate potato crop evapotranspiration (ETa) under sprinkler irrigation on the sandy loam soil. The field experiment was conducted during the 2018 and 2019 growing seasons at the Navajo Farms within the Navajo Agricultural Products Industry, Farmington, NM. Two irrigation scheduling methods were evaluated as FAO-56 approach evapotranspiration-based scheduling and soil moisture sensing irrigation scheduling. Sentek capacitance soil moisture probe was used across four commercial potato fields in each year after calibration to the soil texture just after installation. Crop Evapotranspiration values estimated by the water balance method and the two-step approach were compared to the satellite-based models used in OpenET. The results showed that the potato’s effective rooting zone is the upper 40 cm soil layer. Potato plants extracted more than 50% of total water from the upper 15 cm of the soil profile and about 85% from the upper 40 cm of the soil profile. Little water amount was extracted from the 40–60 cm soil water. Potato crop seasonal evapotranspiration averaged 580 to 645 mm in 2018 and 2019, respectively. The Two-step approach ETa values of 795.5 and 832.7 mm in 2018 and 2019, respectively, were higher than the soil water balance estimated ETa. The satellite modeled ETa varied with field and years and ranged from 437 to 759 mm and averaged 570.4 mm for the 2016–2020 period. Soil moisture probe-based irrigation scheduling improved irrigation water management and the irrigation water use of potatoes in the semiarid climate.

1. Introduction

Potato (Solanum tuberosum L.) is adapted to different climates and worldwide grown and is the fourth-largest crop produced worldwide after rice, wheat, and maize (FAOSTAT, 2021). Potato is a shallow rooting crop and is therefore very sensitive to soil and watering regimes as deep and well-drained soils are favorable for potato production [1,2,3,4,5,6,7,8]. Watering regimes affect potato crop growth, seasonal water use, total tuber yield, marketable yield, and grades. Potato water use is a function of the relative maturity dates, climate, and management practices. A large range of potato seasonal irrigation requirements is reported in the literature depending on the seasonal precipitation and it is temporal distribution [9,10,11,12,13,14,15,16]. Heritage potatoes’ water requirements vary from 610 and 611 mm and the modern potatoes water requirements varied from 491 to 550 mm in New Zealand [17].

Crop evapotranspiration is the main source of water losses in the continuum soil-plant-atmosphere. Chen et al. [18] pointed out that potato evapotranspiration (ET) strongly depended on management practices, irrigation level, and soil type. Potato ET was 445.2 mm in Erzurum-Turkey [19], 445–683 mm in semiarid climatic conditions of Turkey [20], 413.2 under drip irrigation in a loam soil while it was 362.1 mm in clay soil in Valenzano, Italy [21], 375.7–511.4 mm under arid climate in Iraq [22], 331.5 mm under rainfed condition in South-eastern Canada [23], 411–448 mm in Sicily, Italy [24], and 216.5–249.3 mm under rainfed production in Gansu Province of China [18]. Ferreira and Carr [25] reported potato variety Desirée ET that varied from 150 to 550 mm and which is dependent on the watering regime. Similar findings are reported by Paredes et al. [26] who found that rainfed potato ET varied from 186 to 219 mm under rainfed conditions while it varied from 295 to 484 mm under different irrigation regimes. Aksic et al. [27] reported that potato seasonal evapotranspiration varied with irrigation depths and ranged from 288.1 to 522.1 mm with the lowest value obtained under rainfed conditions while the highest value was obtained under the well-irrigated treatment.

Soil water monitoring is critical for efficient and sustainable irrigation management. El-Shafie et al. [28] investigated soil water dynamic under furrow irrigated potato in Kalubia Governorate with a clay loam soil, Egypt, and found that no large variation in soil moisture content below the upper 20 cm soil layer due to frequent irrigation and stayed between 0.33 to 0.29 m³m⁻³ for the layers 20–30, 30–40 and 40–50 cm over the season under 0.7 and 1.0 m gate spacings. Paredes et al. [26] found larger potato root densities in the top 30 cm soil layer with the effective root depth to be 0.50 m in silty loam soil of Bari (IAM-Bari), Valenzano, Southern Italy. Yamaguchi and Tanaka [29] observed that 90% of potato root was distributed in the upper 35–38 cm soil layer. In coarse sand, most of the potato roots accumulated in the surface 0–30 cm soil layer compared to the loamy sand, and sandy loam soils while in a loamy sand soil, higher root density was found in the deep soil profile (30–70 cm) compared to the sandy loam and coarse sand as found by Ahmadi et al. [30] who highlighted that the highest root length density existed in the top 30–40 cm of the ridge below which is decayed sharply. About 80% of potato roots are located within the upper 15 cm soil layer and a low root length density was observed within 24 and 36 cm from the top of the ridge under shallow water table in Hastings, Florida [31,32]. Opena and Porter [33] reported that 85% of the root length was concentrated in the upper 30 cm soil layer. Stieber and Shock [3] related that the ideal sensor locations for monitoring soil water status and scheduling irrigations are offset 0.15 m from the center of the hill and 0.1 to 0.2 m deep.

Potato is one of the most important crops grown in the United States with 21.338 million tons of total production in 368,952 hectares and an average yield of 57,871.8 kg/ha for a value of $3.65 billion in 2020 and Idaho and Washington State are the two producing potatoes in the United States. Potatoes are therefore economically very important in the US and need attention in terms of water management sustainability. Due to the sensitivity of potato crops to water stress and the limited available water resource under changing climate, and the lack of information on the effective rooting depth of potato crops in the study area, this study aimed to (1) investigate soil water dynamics, (2) determine the effective root zone and (3) estimate crop evapotranspiration of potato under limiting water conditions in the sandy loam soil in the commercial potato fields.

2. Materials and Methods

2.1. Study Area

This study was conducted at the Navajo Mesa Farms which is a contractor within the Navajo Agricultural Products Industry (NAPI) located in San Juan County in northwestern New Mexico (Long. 36.676943, Lat. 108.260844, Elev. 1830 m) during the 2018 and 2019 crop growing seasons (Figure 1). Minimum temperature (Tmin), maximum temperature (Tmax), average temperature (Tmean), minimum relative humidity (RHmin), maximum relative humidity (RHmax), average relative humidity (RHmean), wind speed (u2), and solar radiation (Rs) were collected on a daily basis from an automated weather station installed at the New Mexico State University Agricultural Experiments Station (Latitude 36.69′ North, Longitude 108.31′ West) which is located within NAPI exploitation domain. The weather station was about 0.5 to 3 km from the potato fields, depending on the potato field under consideration, and the year. The soil at the study area is a well-drained sandy loam with the moisture at field capacity of 29.7% and the moisture at wilting point of 15.2%. Soil pH averaged 8 and the soil organic matter content was below 1%.

2.2. Potato Crop Management

All fields were ripped to break the compacted soil layers due to the heavy equipment operations during the previous growing season. The plots were thereafter disked, harrowed and the seedbed raised. The plots were treated with Vapam HL (sodium methyldithioearbamate) at the applied rate of 374 L/ha. Vapam HL is a combination soil fumigant to control weed soilborne diseases, symphylids, nematodes. Chip potato seed pieces were cut to 71 g average size and planted with 23 cm seed spacing on April 16 in 2018 and 2019. Five and four different potato cultivars were planted in 2018 and 2019, respectively. One week after planting, the plots were tilled using a dammer diker to reduce surface runoff and for water conservation. All plots were managed according to predetermined operation programs in terms of weed, insects, fungi, fertilizer, and water management. At crop maturity, potato vine was killed in early September using the combination of Reglone, Brimstone, and Rainier-EA.

2.3. Irrigation Water Management

Irrigation scheduling was based on root zone soil moisture sensor data monitored throughout the potato growing season. Sentek capacitance probe 55 cm long was installed within the most dominant soil type in a plot and a total of five probes were installed in five different plots on 1 June 2018, and four probes on 14 May 2019, with on-site calibration using site specific calibration equations from Sentek’s library. The delay in sensor installation in 2018 was due to the delay in equipment delivery from the manufacturer. The Sentek soil moisture probe incorporates 6 sensors at 5, 15, 25, 35, 45, 55 cm below the soil surface, monitoring soil moisture content at these soil depths. The moisture probe is paired with a telemetry device to send data to a website designed by the manufacturer and accessible on computers and smart cell phones. The soil moisture content was monitored on 30-min basis. Hourly and daily average soil moisture content is derived from the 30-min basis data and used for crop evapotranspiration and irrigation depth estimation. Because potato plants are very sensitive to water stress, the irrigation trigger point was set as 35% soil water depletion of the total available water (TAW) as shown in Figure 2. Irrigation water was delivered through a center pivot sprinkler irrigation system. During the 2019 growing season, Sentek soil moisture probes were installed just after dammer diker tillage (Figure 3A) and removed from the field by the end of August, before vine kill while they were installed at the end of May during the crop active growth period (Figure 3B) and removed out of the field just before harvest (Figure 3D). The soil moisture probes were removed earlier in 2019 due to the damage caused to two telemetry devices that were broken during on-ground vine killing process.

2.4. Estimation of Potato Crop Actual Evapotranspiration by the Water Balance Method

The Sentek capacitance probe soil storage water measured data were used in the soil water balance equation for crop actual evapotranspiration (ETa). A general soil water balance equation is:

$$\mathrm{P}+\mathrm{I}+\mathrm{U}=\mathrm{R}+\mathrm{D}\pm \Delta \mathrm{S}+\mathrm{ETa}$$

(1)

where P = effective rainfall (mm); I = irrigation water applied (mm); U = upward vertical soil moisture flux from below the crop root zone (mm); R = surface run-off (mm); ΔS = change in soil moisture storage in the crop root zone (mm); and D = water lost through deep percolation, vertically downward from the root zone (mm). Assuming that the upward flux was negligible because of the water table that is deep below the soil surface, and R is negligible due to the very low rainfall amount under the arid conditions and the accurate irrigation scheduling, the equation is reduced to the following form for calculating ETa:

$$\mathrm{ETa}=\mathrm{P}+\mathrm{I}-\mathrm{D}\pm \Delta \mathrm{S}$$

(2)

Deep percolation was estimated through the weekly water balance method as the depth of water drained below the 60 cm soil upper layer. Potato seasonal actual evapotranspiration was estimated by summation of the weekly ETa from potato planting to vine killing.

2.5. Estimation of Potato Crop Actual Evapotranspiration by the FAO-Penman Monteith Method

Potato actual crop evapotranspiration was estimated according to the equation proposed by Jenson [34] and Allen et al. [35]:

$$\mathrm{ETa}=\mathrm{Kc} \times \mathrm{ETo}$$

(3)

where ETa = daily actual evapotranspiration (mm), Kc = daily crop coefficient, and ETo = Penman-Monteith grass reference evapotranspiration (mm).

Potatoes were grown under non-limiting water and fertilizer conditions, and the standard FAO crop coefficient values were used [35]. Potato crop coefficients developed under a standard climatic condition [35], as 0.5, 1.15 and 0.75 for the initial, mid-season and late-season were also used to estimate potato ETa for the study period.

Daily grass-reference ET was computed using the standardized ASCE form of the Penman–Monteith (PM-ETo) equation [36]:

$$\mathrm{ETo}=\frac{0.408∆\left(\mathrm{Rn}-\mathrm{G}\right)+\left(\mathsf{\gamma }\mathrm{Cn} \mathrm{u}2/\left(\mathrm{T}+273\right)\right)\left(\mathrm{es}-\mathrm{ea}\right)}{∆+\mathsf{\gamma }\left(1+\mathrm{Cd} \mathrm{u}2\right)}$$

where ETo is the reference evapotranspiration (mm day⁻¹), Δ is the slope of saturation vapor pressure versus air temperature curve (kPa °C⁻¹), Rn is the net radiation at the crop surface (MJ m⁻² d⁻¹), G is the soil heat flux density at the soil surface (MJ m⁻² d⁻¹), T is the mean daily air temperature at 1.5–2.5 m height (°C), u2 is the mean daily wind speed at 2 m height (m s⁻¹), es is the saturation vapor pressure at 1.5–2.5 m height (kPa), ea is the actual vapor pressure at 1.5–2.5 m height (kPa), es − ea is the saturation vapor pressure deficit (kPa), γ is the psychrometric constant (kPa °C⁻¹), Cn and Cd are constants with values of 900 °C mm s³ Mg⁻¹ d⁻¹ and 0.34 s m⁻¹. The procedure developed by Allen et al. [35] was used to compute the needed parameters.

2.6. Potato Crop Actual Evapotranspiration Retrieved from OpenET, a Satellite Based Water Date Resource

Potato monthly evapotranspiration was retrieved from The OpenET (https://openetdata.org/ accessed on 15 January 2022) which is a community-driven effort that is building upon the advances to develop an operational system for generating and distributing ET data at a field scale using an ensemble of six well-established satellite-based approaches for mapping crop evapotranspiration [37]. OpenET has an operational system for field-scale ET mapping across the western U.S. and it provides access to both spatially continuous gridded datasets and choropleth maps that summarize data to individual field boundaries [37]. Twenty potato fields were located and selected each year from 2016 to 2020 from the Navajo Agricultural Products Industry’s domain using OpenET website (Figure 4). The current ensemble of ET models included in OpenET is composed of Atmosphere-Land Exchange Inverse/Disaggregation of the Atmosphere-Land Exchange Inverse (version 0.0.27) [38,39], Mapping Evapotranspiration at High Resolution with Internalized Calibration (version 0.20.15 [40,41,42], Surface Energy Balance Algorithm for Land using Google Earth Engine (version 0.2.1) [43,44], Priestley-Taylor Jet Propulsion Laboratory (version 0.2.1) [45], Satellite Irrigation Management Support (version 0.0.20) [46,47], and Operational Simplified Surface Energy Balance (version 0.1.5) [48,49]. For operational purposes, for a selected year, twenty potato fields were located and the monthly evapotranspiration data was retrieved. This process was repeated for each year from 2016 to 2020. In the NAPI and Navajo Mesa Farms are commercial potato producers with the large planted areas, field preparation usually starts by mid-March with soil preparation, pre-irrigation, fumigation (under conventional production), and planting starting early April and ending mostly ends April, with harvest late September–early October before the first fall frost. In the present study, potato growing period was taken as from 1 April to 30 September even if planting is echeloned over April and harvest from 15 September up to 15 October. Planting and harvesting mostly depend on resource availability and their occurrence may vary from year to year. Monthly crop ET of the selected fields were retrieved and monthly average ET, seasonal ET, and the average seasonal ET of potato were estimated regardless the potato variety, effective planting date, and the effective harvest date.

3. Results

3.1. Weather Conditions during the 2018 and 2019 Growing Seasons

Daily weather conditions during the experiment period 2018–2019 are presented in Figure 5. The maximum, minimum, and average temperatures increased from January to the maximum values mid-July and decreased thereafter to the minimum values at the end of December of each year. The monthly average air temperature varied from −14.92 °C to 27.45 °C in July and decreased to the lowest value of −7.22 °C in December. Overall, the weather was warmer during 2019 with an annual average mean temperature of 12.17 °C compared to 2018 with an annual mean temperature of 10.85 °C. Similarly, the 2018 growing season (April–September) average daily mean temperature was 20.43 °C against 18.51 °C in 2019. Potato growing season air relative humidity averaged 30.21% in 2018 while it was 40.48% in 2019 reflecting greater rainfall received in 2019 compared to the 2018 total precipitation (Figure 5). Seasonal wind speed averaged 2.38 and 2.18 m/s while the solar radiation averaged 25.17 and 23.88 MJ/m² in 2018 and 2019, respectively. Total seasonal precipitation was 74.42 mm in 2019 against 48.37 mm in 2018 representing 53.9% more precipitation in 2019 compared to the 2018 potato growing season.

3.2. Dynamics of Soil Water at Different Depth of the Soil Profile (0–60 cm)

The variation in the soil water storage is shown in Figure 6 for the 2018 growing season and Figure 7 for the 2019 growing season. Soil water varied more frequently with the 0–10 cm, 10–20 cm et 20–30 cm soil layers compared to the deep layers 30–40 cm and 40–50 cm and 50–60 cm. The drop in soil water in the deep soil layers as shown in Figure 61,2 is due to sensor relocation. Sensors 1 and 2 were not properly responding and were moved to new positions for better and more precise water management. The topsoil layer (0–10 cm) water varied from 5 to 30 mm with a frequent increase at each irrigation and or precipitation event. The frequent and large variation of the soil water of the top two layers might be due to soil water evaporation and the potato crop uptake. Soil water within the 40–50 cm and 50–60 cm showed slightly negligible variation and oscillated around 21 and 22 mm in field 1, 24 and 19 mm in field 2, 34 and 33 mm in field 3, 28 mm, and 31 mm in field 4 and 17 and 19 mm in field 5, in 2018, respectively. The soil water in the layer 30–40 cm in all fields in 2018 showed a decreasing trend toward the end of the growing season probably due to the deep root water uptake. The soil water within the bottom three soil layers showed little variation however, there was an abrupt increase in soil water storage within those layers in the field 2, 3, and 5 on August 22, (fields 4 and 5) and August 27 (Field 2) in 2018, was the response to high applied irrigation rate as a strategy to increase soil water storage in the case winter small grains (wheat, triticale, and barley) will be grown in rotation with potatoes. Irrigation water is usually cut off by October 15 until mid-march exposing the winter crops to drought as the winter is always dry in the arid climate conditions of the study area.

With early soil moisture sensors installation before potato sprouting in 2019, soil water showed variation mostly within the 0–20 cm soil layer during the potato initial growing stage (Figure 7). Throughout the growing season, the top 0–10 cm soil layer showed the lowest soil water storage which varied from 4 to 21 mm, from 3 to 26 mm, from 6 to 16 mm, and from 11 to 24 mm in the fields 1, 2, 3, and 4, respectively. The soil water in the second soil layer 10–20 cm varied from 13.5 to 27 mm in field 1, from 13.5 to 28 mm in field 2, from 11.4 to 26 mm in field 3, and from 30 to 29 mm in field 4 (Figure 7). Large variation of the soil water in the two soil layers started after the actively growing stage of potato crops. Soil water variation in the 20–30 cm soil layer, showed slightly lower frequency and lower magnitude and was within 27–36 mm, 26.5–38 mm, 20–30 mm, and 27–34 mm in fields 1, 2, 3, 4, respectively. Soil water varied very slightly within the 30–40 cm soil layer within the range 28–38 mm in the field 1, 36.5–39 mm in field 2, 27–34 mm in field 3, and 31.2–35.5 mm in field 4 (Figure 7). The soil layer 40–50 cm showed a decreasing trend in the soil water from planting to crop maturity. The soil layer 40–50 cm and 50–60 cm showed negligible variation in soil water on a season basis in 2019.

The monthly total applied irrigation for the five fields in 2018 and the four fields in 2019 averaged 22.02, 17.78, 107.18, 233.68, 195.07, 144.78, and 45.72 mm in March, April, May, June, July, August, and September (Figure 8). The average seasonal applied irrigation amount was 777.23 ± 35.51 mm. The applied irrigation amount in March is a preplant one and helps improve soil work and seedbed preparation.

The results of this study are in agreement with the findings of El-Shafie et al. [28] who indicated that the top soil layers showed more variation in soil water during the potato growing season. As the soil water dynamics is the combined activity of soil water evaporation and crop water uptake, the results of this study might be correlated with potato plant root distribution and activity even if root biomass, length, or surface were not determined. Munoz-Arboleda et al. [31] found 94% of potato root length in the upper 0–24 cm soil layer corresponding to 92.4% of the total root surface area. Opena and Porter [33] found approximately 85% of potato root length concentrated in the upper 30 cm soil layer. Most of the potato root was found in the upper 30 cm [50,51]. Stalham and Allen [51] found that 40–73% of potato total root length was confined in the top 30 cm soil layer. Wang et al. [52] pointed out that the root weight density describes better the potato root system of potatoes due to the thinness of the roots even if the root-length density is most commonly used to describe the extent of the root systems of agricultural crops.

3.3. Potato Seasonal Evapotranspiration by Different Approaches

The results of the soil balance equation using the soil water data for the 2018 and 2019 seasons showed that potato ET varied from 534 to 639 mm in 2018 and from 560 to 681 mm in 2019 and averaged 580 ± 38 and 645 ± 57 mm in 2018 and 2019, respectively. The 2019 seasonal potato ET was therefore 11.2% higher than the 2018 seasonal potato ET. The two-step approach ET was 795.5 and 832.7 mm in 2018 and 2019, respectively, and were higher than the soil water balance estimated ET. The Penman-Monteith method estimated ET was 37.2 and 29.1% higher than the soil balance method ET in 2018 and 2019, respectively.

The satellite modeled ET varied was summed on the month basis and season basis for each field and year for the 2016–2020 period. Monthly satellite-based ET averaged 21.45, 61.38, 151.28, 176.39, 126.90, and 63.50 mm for April, May, June, July, August, and September, respectively (Figure 9). Minimum ET was obtained in April when potatoes barely sprout. No transpiration occurred and soil water evaporation is predominant. The highest evapotranspiration in June corresponds to the full canopy cover while the low ET in September is mostly soil water evaporation as most potato vines are tilled by August. However, irrigation after vine kill is recommended to avoid cracks in the soil mostly during the late-season stage and potato tuber skin set and strengthening and proper potato maturity. Rondon et al. [53] and Clough et al. [54] suggested that applying 2.5 mm/day through a center pivot irrigation system from vine kill to harvest decreased P. operculella tuber damage and did not increase fungal or bacterial diseases. Satellite retrieved seasonal ET varied from 493.02 to 668.78 mm in 2016, from 475.10 to 622.80 mm in 2017, from 437.01 to 625.28 mm in 2018, from 484.06 to 617.09 mm in 2019, and from 573.34 to 758.76 mm in 2020 and averaged 571.35, 544.88, 540.96, 546.69 and 648.08 mm in the respective seasons. The overall average seasonal ET for the 2016–2020 period was 570.39 mm.

The comparison of the three methods of potato monthly evapotranspiration is shown in Figure 10. The peak monthly was obtained in June in 2018 with a monthly ETa of 177 mm by the soil moisture monitoring method and July in 2019 with the monthly ETa of 212 mm by the soil moisture method. Considering the water balance method as the actual potato ETa, the OpenET underestimated monthly ETa from 3 to 16% in 2018 and from 9 to 13% in 2019 during the May-August period, considered as the critical period of potato growth period in the study area as potato vine kill occurs usually the first week of September. The FAO two step method overestimated potato monthly ETa from 8 to 40% in 2018 and from 8 to 65% in 2019 during the May-August period (Figure 10). Over the growing season, OpenET underestimated potato ETa by only 1.2% in 2018 and 8.5% in 2019 while the two step FAO method overestimated potato ETa by 37% and 29.2% in 2018 and 2019, respectively.

Using the FAO Penman-Monteith method, Djaman et al. [55] found potato seasonal ET of 857.7 and 869.3 mm in two consecutive seasons in the same study area. Alva et al. [56] reported potato seasonal ETa of 825 mm in Benton County, WA under a Quincy fine sand soil. Lower potato seasonal ET of 475.2 mm was reported by Kiziloglu et al. [19] in Erzurum-Turkey. Ati et al. [22] found potato seasonal ET to vary between 375.7 and 511.4 mm under arid climate in Iraq while under the hot and dry climate in Spain, Ferreira and Carr [25] obtained potato seasonal ET that varied from 150 to 550 mm as a function of watering regimes. The high and inaccurate crop ET by the Penman-Monteith method might be due to the adoption of the FAO crop coefficients. The crop coefficient is an integrated factor that is assumed to account for the crop’s physiological conditions, irrigation method, management practices, and soil and climate conditions [57]. Potato water use varies with management practices and irrigation levels [18]. Potato ET was 413.2 ± 15 mm under drip irrigation in a loam soil while it was 362.1 ± 16 mm in clay soil in Valenzano, Italy [21]. Well irrigated potato seasonal evapotranspiration was 445.2 mm in Erzurum-Turkey [19] and varied from 375.7 to 511.4 mm under arid climate in Iraq [22]. Under hot and dry climate in Spain, potato variety Desirée evapotranspiration was a function of the applied irrigation amount and ranged from 150 to 550 mm [25]. Erdem et al. [20] reported a seasonal potato ET range of 445–683 mm in semiarid climatic conditions of Turkey. Fully irrigated potato seasonal ET was 355 mm and 484 mm in two consecutive seasons under the Mediterranean condition in Italy [26]. Ierna and Mauromicale [24] reported that well-irrigated potato seasonal ET was 411 mm in Sicily, Italy. The use of FAO crop coefficients overestimated crop ET of potatoes [55,58], cotton [59], orange orchard [60], citrus orchards [61]. Mata-González et al. [62] showed the inappropriateness of using FAO-56 Kc values which overestimated crop ET of different crop types in arid ecosystems, where plants do not have high Leaf Area Index (LAI) or little stomatal resistance to water loss. Similar findings were reported by Zanotelli et al. [63] who showed a discrepancy between apple orchard Kc values retrieved from FAO and their values based on eddy covariance towers placed in south Italy. It is therefore important de develop local crop coefficients for potatoes to improve water management for system sustainability [64,65]. Mhawej et al. [65] found better irrigation management using the satellite-based adjusted single coefficient in California, USA. Shauer and Senay [66] demonstrated the useful application of historical Landsat ET to produce relevant water management information in the central valley of California. Satellite-based ET has been successfully retrieved and used to improve water management under different environments [32,40,67,68,69,70,71,72].

4. Conclusions

Field experiments were conducted to investigate the variation in the soil water storage in different soil layers (0–60 cm) under a well-irrigated potato and potato crop evapotranspiration was estimated by three different methods (soil water balance method, FAO Penman-Monteith method, and satellite retrieved measurement). The results showed more variation in the soil water in the 0–10 cm layer followed by 10–20 cm, 20–30 cm, and 30–40 cm. Very little variation in the soil layer 40–60 cm was observed showing that potato’ effective rooting zone should be considered as 40 cm for better and sustainable irrigation water management. The seasonal ET was averaged 612.5 mm by the soil water balance method, 645.8 mm measured by satellites, and 814.1 mm by the Penman-Monteith method for the 2018–2019 period. Assuming the soil water balance method as the most accurate ET estimation method, the satellite method underestimated potato seasonal ET by 11.2% while the Penman-Monteith method overestimated potato ET by 32.9%. Considering the soil and plant population and growth characteristics variation across the same field and the advection at the edge of the field, the satellite retrieved ET might be more reasonable compared to the soil water balance method based only on a very small section of the field with the soil moisture probes installed at the more representative part of the field in terms of plant density. The Penman-Monteith method overestimated potato seasonal ET and might be used if locally developed crop coefficients are available for the study area. The findings of this study highlight that water resource planners should consider the importance of remote sensed crop ET in combination with in situ monitoring using soil moisture sensors to improve agricultural water management at the field and regional scales.